Bring back moreStyleSheets as adoptedStyleSheets

After discussions regarding https://github.com/WICG/construct-stylesheets,
we are actually interested in having a way to style TreeScopes
with constructed CSSStyleSheet. This CL brings back moreStyleSheets,
renaming it as adoptedStylSheets (was deleted in crrev.com/c/1125741
and crrev.com/c/1155286). No additional behavior change from previous
implementation. Other changes to the spec will be added in separate CL.

Draft spec at: https://wicg.github.io/construct-stylesheets/
Previous implementation CLs: crrev.com/c/925763, crrev.com/c/938872

Bug: 807560
Change-Id: I65b4de496c0a70566d293374249816643456d8b9
Reviewed-on: https://chromium-review.googlesource.com/1197002
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#587993}
16 files changed