Implement Document.createCSSStyleSheet

We previously implemented constructable CSSStyleSheet synchronously,
but discussions in  https://github.com/WICG/construct-stylesheets/issues/2
have gravitated away from that and we are now interested in asynchronous
creation of CSSStyleSheet.

This CL added Document.createCSSStyleSheet that returns a
Promise<CSSStyleSheet>, while also removing the exposed CSSStyleSheet
that was previously exposed. Note that because CSS parsing is still
done on the main thread, this function is actually still blocking.

The previously implemented CSSStyleSheet is changed to only produce
empty CSSStyleSheets in this CL: crrev.com/c/1126898

Bug: 807560
Change-Id: I9f9d17ae04829ff399ae384f8b3a6d97a3b0613b
Reviewed-on: https://chromium-review.googlesource.com/1126754
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Takayoshi Kochi <kochi@chromium.org>
Reviewed-by: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575600}
7 files changed