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This cl adds an autotest private API which can be used by a follow up cl
to add tast test to measure the photo transition animation perf.
This API will wait to run several photo transition animations.

Bug: b/156854495
Test: new unittests
Change-Id: Icc9d9ab6e83657e160c6228d1702237ac9eba56e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2163146Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770725}