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The present CL is a new way to serve the same purpose as [1]. As
SplitViewDragIndicators::WindowDraggingState has values called
kFromOverview and kFromTop, I think it is most intuitive to add one
called kFromShelf, with the caveat that the difference between kNoDrag
and kFromShelf is all about animations. Compared to [1], I think it is
nice to avoid adding a whole extra member variable.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/1894754

Test: ash_unittests DragWindowFromShelfControllerTest.SplitViewDragIndicatorsWindowDraggingStates
Test: ash_unittests SplitViewDragIndicatorsTest.SplitViewDragIndicatorsVisibility
Fixed: 1020349
Change-Id: I69aefae3a68579d2f0d0c61a8d8f5588689d0b13
Bug: 1020349
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1898771
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#713270}