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The present CL addresses system instability after a display is added (or
you end mirror mode) while in tablet split view with empty overview.
OverviewSession::OnDisplayAdded calls OverviewController::EndOverview
which fails in case of tablet split view with empty overview. Thus,
overview stays active, but there is no overview grid for the added
display. Then you certainly can get a crash, because a lot of code
assumes that when overview is active, there is an overview grid for each
display. The present CL ensures that overview will actually end when a
display is added.

Test: All/SplitViewOverviewSessionTest.ExitAndReenterMirrorModeWithEmptyOverview/?
Change-Id: Ib9e359e2b231db0f531f7f1de4af9c5af0b2d408
Fixed: 1124481
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2390964Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#805437}