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Painted scrollbars are used for overlay scrollbars on MacOS. Thumb
movement would cause a commit on every scroll change because the update
rect (set via Layer::SetNeedsDisplay) would change. This patch ignores
the update rect in PaintedScrollbarLayer::Update and uses part
invalidation for causing commits. With this change, scroll offset
changes do not cause commits.

Bug: 1048384f
Change-Id: I79bac7fbd9756cb71af7c3b83be3df824dd7a5cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078847Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745544}