CSS: only add [is=..] selector if element is type-extension

[ebidel]

Blink has recalc style/paint issues with complex attribute selectors...

In Chrome stable on chromestatus.com, I see large repaints on scrolling. The culprit is from a complex attribute selector that the styling polyfill produces:

chromedash-metadata ul li[selected],
[is=chromedash-metadata] ul li[selected] {
  font-weight: 600;
  color: rgb(54, 101, 151);
}

Note: turning off the font-weight:600; alone speeds up the scrolling.

It would be killer if the polyfill only added ([is=chromedash-metadata]...)-type selectors if the element is actually a type-extension. This will be an easy
perf boost for users.

[sorvell]

Should be addressed as of googlearchive/platform@4e83085.

[ebidel]

👍 Actual commit is googlearchive/platform@515c583