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* Make SetScreenInstance() return the previous screen instance.  This
  will easily let callers save/restore screen instances.
* Use ProcessDisplayChanged() in an applicable spot.
* Stop overriding GetDisplayMatching() in DesktopScreenWin.  The parent
  class definition is more accurate, and this looks to be an ancient
  artifact.
* Replace some DISALLOW_COPY_AND_ASSIGN.
* Various other stuff

Bug: none
Change-Id: Ic6d545c828e5ec68f015cc7677b6ce74cc14e286
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2145631
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759950}