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This CL implements desks reordering with drag & drop. The main changes
are listed as below:

- Added response methods for mouse pressed, drag and released events in
  desk preview to trigger drag and drop.
- Added *DragDesk methods in desks bar view to handle drag and drop.
- Added a new class DeskDragProxy to create a proxy of desk being
  dragged.
- Added a method PerformMiniViewReorderAnimation to animate the
  movement of desks while reordering.

Bug: 1154038
Change-Id: I7633970a94d69e1326cf09eaede3929ca0acb811
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2602859
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843858}