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To be consistent with tablet splitview behavior, we decided to do the
same when in overview dragging an overview window to snap to a position
that already has a snapped window in place: the previous snapped window
will be put back into overview.

Bug:991695
TEST: SplitViewOverviewSessionInClamshellTest.BasicFunctionalitiesTest

Change-Id: I3411fff3257146bdcd6b4344206a7bbb1fbd6340
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1765444
Commit-Queue: Xiaoqian Dai <xdai@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690976}