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This makes it possible to store the WorkspaceController as a property
of desks containers instead of tracking each in the RootWindowController

BUG=866622
TEST=ash_unittests pass.

Change-Id: I7f47c3c9adecbfb03a4b374bc36f57d2ec3dbc1f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1553715Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Commit-Queue: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#648387}