Backport Bug 1438487 - Restore the Browser Toolbox using session store (firefox-devtools#5521)

Author: jasonLaster

src/test/mochitest/browser_dbg-chrome-create.js

@@ -44,6 +44,7 @@ registerCleanupFunction(function() {
 add_task(async function() {
   // Windows XP and 8.1 test slaves are terribly slow at this test.
   requestLongerTimeout(5);
+  await pushPref("devtools.chrome.enabled", true);
   await pushPref("devtools.debugger.remote-enabled", true);
 
   gProcess = await initChromeDebugger();