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This prepares for blink::ScrollbarDisplayItem (for CompositeAfterPaint
composited scrollbars) which will share ownership of cc::Scrollbar
with cc's painted scrollbar layers.

Also make sure that cc::Scrollbar is accessed from the main-thread
only.

Bug: 931486
Change-Id: If64b14d8d917272873d35f33302ecbb66b20126b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884577Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710872}