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If there are no visible app windows, restoring app window wasn't re-registering
events because OnWindowVisiblityChanged notification wasn't generated for root window.

Add unit test for going from visible to hidden to occluded.

Bug: 813093
Change-Id: I11b5fd7c7dc104ec74699c8e66aa06d9954a5977
Reviewed-on: https://chromium-review.googlesource.com/c/1326621
Commit-Queue: David Bienvenu <davidbienvenu@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611322}