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This change implements percent based scrolling for mousewheels on linux
as part of the scrolling roadmap. If percent based scrolling is enabled,
mousewheel scrolls will now be percent based scrolls. This is already
implemented on Windows.

Bug: 1101535
Change-Id: I91cf774a2dc3fe5e129939c64a0e5bc645b8fe9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354663Reviewed-by: Timothy Dresser <tdresser@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Rahul Arakeri <arakeri@microsoft.com>
Commit-Queue: Sahir Vellani <sahir.vellani@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#804058}