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Keyboard arrow scrolling and scrollbar button scrolling should also
scroll a percentage of the scrollable area when percent based scrolling
is enabled.

This CL adds plumbing from the base::Feature to blink, and uses 1/8th as
the percentage to use for directional scrolls via keyboard arrows or
scrollbar buttons. Adds a test suite that runs scrollbar tests in
percentage mode. Keyboard is currently handled on the main thread so we
just use the runtimeFlag toggling via javascript and eventSender.

Bug: 1008153
Change-Id: I7ea7c5471298187da757cf22748620c4accaf192
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2057501
Commit-Queue: Daniel Libby <dlibby@microsoft.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743455}