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When a desk is activated, this CL adds a horizontal
movement from the starting (current) desk to the
ending (to be activated) desk on all displays, since
a desk spans all displays.

Demos: https://bugs.chromium.org/p/chromium/issues/detail?id=866622#c24

BUG=866622
TEST=Manual, added tests to increase code coverage in
     the case of multi displays.

Change-Id: I901b000a6ff89173d70e3ab07cbb7656c34d8357
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1591000
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657376}