Remove adopted stylesheets when adopting to a different document

When adopting a subtree to a different document, we'll remove all the
adopted stylesheets in that subtree, instead of keeping them and then
ignoring them in style calculation.

Relevant discussion: https://github.com/WICG/construct-stylesheets/issues/23

Bug: 807560
Change-Id: I7ea6c869892cabd9d3b5a765f26364e51b1419b4
Reviewed-on: https://chromium-review.googlesource.com/c/1415068
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/master@{#623660}
6 files changed