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Currently, the decision on whether a scroll should be animated or not
occurs in InputHandlerProxy. This makes the compositor scroll interface
more complicated than it needs to be and exposes details about scrolling
that should be encapsulated in the compositor.

This CL moves the |smooth_scroll_enabled| from an ad hoc bit in the
InputHandlerProxy into a real setting in LayerTreeSettings. We then
rearrange the public interface of the compositor input handler so that
we only have a ScrollUpdate method. This method now checks whether a
given scroll should be animated or not and delegates to the old ScrollBy
or ScrollAnimated methods, based on the answer. Finally, we remove some
of the divergence between animated and non-animated paths in
InputHandlerProxy so that both kinds of scrolls are handed in the same
way.

Bug: 1016229
Change-Id: I2143cf27aeb6aca905b59b817fe8e3f1b14b7858
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1981711
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Majid Valipour <majidvp@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729938}