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This CL allows AutoSnapController to check if
DragWindowFromShelfController is restoring windows after dragging.
AutoSnapController uses kAnimationsDisabledKey to skip auto snapping
after transient hide & show operations by DragWindowFromShelfController.
But kAnimationsDisabledKey may be used in other locations so we should
not rely on it.
This CL also cleans up auto snapping trigger conditions.

BUG=chromium:1107306
TEST=DragWindowFromShelfControllerTest.HideWindowDuringWindowDragging*
TEST=SplitViewControllerTest.AutoSnapFromMinimizedState

Change-Id: Ibc4f72558db3cf0efa070a77e44d3247b79d9ee7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303337
Commit-Queue: Toshiki Kikuchi <toshikikikuchi@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793002}