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The present CL makes code more consistent between these two workflows:
1. There is already a window snapped in clamshell split view, and you
drag another window from overview and snap it on the other side.
2. There is only one window in overview, and you enter clamshell split
view by dragging and snapping it.

BEFORE: SplitViewController::OnWindowSnapped ends split view mode in
Workflow 1, whereas SplitViewController::OnOverviewModeEnding ends split
view mode in Workflow 2.
AFTER: SplitViewController::OnOverviewModeEnding ends split view mode in
both workflows.

Test: ash_unittests SplitViewOverviewSessionInClamshellTest.BasicFunctionalitiesTest
Change-Id: I698cb047dd2fbd69de8a6db08cfdae91961b58ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1779005
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698970}