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This CL makes the following changes:
- Tab scrubbing will always use 3-finger gestures.
- 4-finger horizontal swipes will switch desks only if
  the flag "--enable-virtual-desks-gestures" is enabled.
- 3-finger horizontal swipes while in Overview will always
  move the highlighter.

This CL is meant to be merged back to M-79.
A follow-up CL will remove the "--enable-virtual-desks-gestures"
flag entirely on M-80, so that virtual desks gestures will always
be enabled going forward.

BUG=1005340
TEST=Tested manually on device, updated existing tests.

Change-Id: I1316e150556a4f2ede79b41dab8ddc3402d39370
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1902232
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713262}