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- This trigger enter/exist with various conditions
number of windows: 2 or 8
tab content: blank or new tab page
device mode: clamshell or tablet mode

This also adds new test api to wait for overview animation state.

Bug: 948324
Change-Id: Ic8d56e12bbd5a60cf93c3add6522cfa128a2764b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1559384
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649336}