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This CL updates some of the scrolling tests with platform specific test
expectations. (Since MockScrollbars no longer exist). The CL also gets
rid of duplicated tests without losing coverage.

Bug: 953847
Change-Id: I0bf10ef0434f9630624f73d9deb3986bd67661cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1925180Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#718331}