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When the MultiDisplayOverviewAndSplitView feature flag is disabled,
during transition to unified desktop mode, the destructors of
BackdropController and ShelfLayoutManager should call
SplitViewController::RemoveObserver on the split view controller for the
old primary root window rather than for the new primary root window. The
present CL addresses the problem by adjusting the timing of when
Shell::GetPrimaryRootWindow stops returning the old primary root and
starts returning the new primary root. The child CL has some LOG(ERROR)
code useful for verifying that the present CL serves its purpose.

Bug: 970013
Change-Id: I81074d4883a7d252d1da6a808976b47ad22cfc32
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1869838
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708745}