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Adds a way to pipe data from WmGestureHandler -> DeskController ->
DeskAnimationBase -> RootWindowDeskSwitchAnimator in preparation for the
touchpad continuous desks gestures feature. This CL will remove the
touchpad animation temporarily, if the feature is on, but will still
switch desks. Done in this CL:

1) WmGestureHandler uses different path if the feature is enabled.
2) DesksController has 3 new functions for WmGestureHandler to use.
3) DesksAnimationBase has Update and End functions.

Test: manual, with and without feature
Bug: 1111445
Change-Id: I08af31452a6614de3d72c42433d1dde8b5704cbd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2388276
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805092}