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We created a StyleSheetList connected to the TreeScope which would
create a second StyleSheetList for the scope-attached sheets which
messed up the active stylesheets causing crashes.

Bug: 819483
Change-Id: Ib24bee538a27a64b5a8206e32334d4324629d5af
Reviewed-on: https://chromium-review.googlesource.com/964326
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Reviewed-by: Takayoshi Kochi <kochi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543654}