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This CL fixes an issue where clicking on a faded out scrollbar on Mac
caused a gesture scroll. This is fixed by querying the opacity of the
PaintedScrollbarLayerImpl and returning a zero delta if the scrollbar
opacity is 0.

Bug: 1054155
Change-Id: I9ae3b8284b1843b99f15e70a66f7336d956aa810
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2068204
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744864}