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When the display compositor is offscreen, and the passed in surface
handle is null, return a stub class for overlay processor. This removes
enable_overlay and could_overlay bools from other overlay processor
classes.

R=Khushal

Bug: 898680
Change-Id: I75f72ba3dfabcc3ab0e2fe80509bb6cad7a74910
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2446463Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: Sunny Sachanandani <sunnyps@chromium.org>
Reviewed-by: Maggie Chen <magchen@chromium.org>
Commit-Queue: weiliangc <weiliangc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816631}