[css-device-adapt] Meta viewport tokenizer should re-use window.open() features tokenizer

[zcorpan]

https://drafts.csswg.org/css-device-adapt/#parsing-algorithm

See https://bugs.webkit.org/show_bug.cgi?id=170548#c36

In whatwg/html#2476 we specified the tokenizer for window.open()'s features argument, and when @cdumez worked on implementing it in WebKit he noticed that the same parser is being used for <meta name=viewport content="...">.

I haven't checked if the same is true in other browser engines, but it seems like a missed opportunity to let these diverge for no particular reason. So I suggest we change css-device-adapt to hook into HTML's features tokenizer, so these are consistent and to reduce implementation complexity/footprint, reduce chances for bugs, and so on. As a side-effect we're more likely to reach interop for meta viewport.