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This fixes an issue when scroll deltas for arrow clicks, shift+click
and autoscrolling was improperly scaled. The fix here is to use the
painted_device_scale_factor (when use-zoom-for-dsf is true). As a
part of this change, 2 VirtualTestSuites have also been introduced.
One of them runs the new tests with DSF=2 on the main thread and the
other one on the cc thread. New test had to be added because the scroll
offsets when DSF=2 (and use-zoom-for-dsf=true) are different compared to
offsets when DSF=1 (and use-zoom-for-dsf=true) both in case of main and
cc scrollbars in some cases (eg: shift+click).

Bug: 1007875
Change-Id: Ib4fbad3004f0ef503420ada6b701432798ec7265
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1848792
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708412}