Clean up the event loop and agent correspondence

This closes #5352, by making event loops and agents 1:1. It also adds an
explicit explanation that event loops/agents and implementation threads
are not necessarily 1:1, apart from the restrictions imposed by
JavaScript's forward progress guarantee.

This also closes #4213, as worklet event loops work fine in this
architecture.

This also closes #4674, as browsing contexts changing event loops is
not a problem in this architecture, since they change agents at the same
time.

Finally, this guards one step in the event loop processing model that
only made sense for window event loops.

Author: domenic

source

@@ -2824,6 +2824,7 @@ a.setAttribute('href', 'https://example.com/'); // change the content attribute
      <li><dfn data-x-href="https://tc39.es/ecma262/#sec-candidate-executions">candidate execution</dfn></li>
      <li>The <dfn data-x-href="https://tc39.es/ecma262/#current-realm">current Realm Record</dfn></li>
      <li><dfn data-x-href="https://tc39.es/ecma262/#early-error-rule">early error</dfn></li>
+     <li><dfn data-x-href="https://tc39.es/ecma262/#sec-forward-progress">forward progress</dfn></li>
      <li><dfn data-x-href="https://tc39.es/ecma262/#sec-invariants-of-the-essential-internal-methods">invariants of the essential internal methods</dfn></li>
      <li><dfn data-x-href="https://tc39.es/ecma262/#sec-execution-contexts">JavaScript execution context</dfn></li>
      <li><dfn data-x-href="https://tc39.es/ecma262/#execution-context-stack">JavaScript execution context stack</dfn></li>
@@ -88916,40 +88917,31 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
 
   </div>
 
-  <h4>Event loops</h4> <!-- <dfn>event loop</dfn> -->
+  <h4>Event loops</h4>
 
   <h5 w-nodev>Definitions</h5>
 
   <p>To coordinate events, user interaction, scripts, rendering, networking, and so forth, user
   agents must use <dfn data-x="event loop" data-lt="event loop" data-export="">event loops</dfn> as
-  described in this section. Each <span>agent</span> has an associated <span>event loop</span>.</p>
+  described in this section. Each <span>agent</span> has an associated <span>event loop</span>,
+  which is unique to that agent.</p>
 
-  <p class="note">In the future, this standard hopes to define exactly when <span data-x="event
-  loop">event loops</span> can be created or reused.</p>
+  <p>The <span>event loop</span> of a <span>similar-origin window agent</span> is known as a
+  <dfn>window event loop</dfn>. The <span>event loop</span> of a <span>dedicated worker
+  agent</span>, <span>shared worker agent</span>, or <span>service worker agent</span> is known as a
+  <dfn>worker event loop</dfn>. And the <span>event loop</span> of a <span>worklet agent</span> is
+  known as a <dfn>worklet event loop</dfn>.</p>
 
-  <p>A <dfn>window event loop</dfn> is the <span>event loop</span> used by <span
-  data-x="similar-origin window agent">similar-origin window agents</span>. User agents may share an
-  <span>event loop</span> across <span data-x="similar-origin window agent">similar-origin window
-  agents</span>.</p>
-  <!-- Ideally user agents process-isolate similar-origin window agents from each other that are not
-       same site. However, this is not the reality in 2019. -->
-
-  <p class="note">This specification does not currently describe how to handle the complications
-  arising from <span data-x="navigate">navigating</span> between <span data-x="similar-origin window
-  agent">similar-origin window agents</span>. E.g., when a <span>browsing context</span> <span
-  data-x="navigate">navigates</span> from <code data-x="">https://example.com/</code> to <code
-  data-x="">https://shop.example/</code>.</p>
-
-  <p>A <dfn>worker event loop</dfn> is the <span>event loop</span> used by <span data-x="dedicated
-  worker agent">dedicated worker agents</span>, <span data-x="shared worker agent">shared worker
-  agents</span>, and <span data-x="service worker agent">service worker agents</span>. There must be
-  one <span>worker event loop</span> per such <span>agent</span>.</p>
-
-  <p>A <dfn>worklet event loop</dfn> is the <span>event loop</span> used by <span data-x="worklet
-  agent">worklet agents</span>.</p>
+  <div class="note">
+   <p><span data-x="event loop">Event loops</span> do not necessarily correspond to implementation
+   threads. For example, multiple <span data-x="window event loop">window event loops</span> could
+   be cooperatively scheduled in a single thread.</p>
 
-  <p class="XXX">As detailed in <a href="https://github.com/whatwg/html/issues/4213">issue #4213</a>
-  the situation for worklets is more complicated.</p>
+   <p>However, because the various worker <span data-x="agent">agents</span> are allocated with
+   [[CanBlock]] set to true, the JavaScript specification does place requirements on them regarding
+   <span>forward progress</span>, which effectively amount to requiring dedicated per-agent threads
+   in those cases.</p>
+  </div>
 
   <div w-nodev>
 
@@ -89386,8 +89378,8 @@ dictionary <dfn>PromiseRejectionEventInit</dfn> : <span>EventInit</span> {
    </li>
 
    <li>
-    <p>Report the duration of the <span>update the rendering</span> step by performing the
-    following steps:</p>
+    <p>If this a <span>window event loop</span>, then report the duration of the <span>update the
+    rendering</span> step by performing the following steps:</p>
 
     <ol>
      <li><p>Let <var>rendering end time</var> be the <span>current high resolution