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The parts of a composited scrollbar (like the thumb, buttons etc) were
expressed as being relative to the origin of the owner scroll layer.
This will cause hit testing for the div scrollbars to fail (except
when the div is absolutely positioned at 0,0). To fix this, we need to
tranform the following coordinates and make them relative to the
scrollbar layer's origin:
a) The pointer coordinates before hit testing (in ScrollbarController)
b) The back,forward,track rect getters in ScrollbarLayerDelegate

Bug: 952314
Change-Id: Ib42ed4df80e30a5c0dc983bc003fb21fb6a42df2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1585017
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#656400}