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Tablet mode has a performance feature which takes a screenshot to fully
occlude all windows while the top window is animating. This CL adds a
test suite to test the feature, including a regression test for a
recently found regression.

Test: added tests
Bug: 1002735
Change-Id: I712ecdb3778f720fa55b6cbcf34773caca0c1f94
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1815229Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#698567}