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This CL implements the unittest for reodering desks by drag & drop.

This CL adds three helper functions:
- LongTapOnDeskPreview
- StartDragDeskPreview
- DragDeskPreviewTo
- DropDeskPreview

The CL adds two unittests: DesksBentoTest.ReorderDesksByMouse and
DesksBentoTest.ReorderDesksByGesture which test

- Trigger drag & drop and reorder desks by mouse.
- Trigger drag & drop and reprder desks by gesture.

Change-Id: I295e393ef4fdcf94a242ba4ecb011bb8f44515d3
Bug: 1154038
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2609587Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Commit-Queue: Xiaodan Zhu <zxdan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843902}