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The present CL refactors overview in preparation for the idea that each
root window will have its own SplitViewController with its own
observers. Each OverviewGrid shall observe the corresponding
SplitViewController. OverviewSession shall observe the
SplitViewController for the primary root window, for the sole purpose of
updating the bounds of the no windows widget.

Test: ash_unittests OverviewSessionTest.* OverviewGridTest.*
Bug: 970013
Change-Id: Iae96ff34eb7e6c3f0fb48eb38d563e3366f72861
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1841776
Commit-Queue: Avery Musbach <amusbach@chromium.org>
Reviewed-by: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703379}