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Frame throttling starts and ends accordingly when window cycling starts
and ends.
Added test helper and unittests.

Bug: 1107201
Change-Id: I98d9ac1675ca8fd6d2eb7bb178c5c8961166dae4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2308131
Auto-Submit: Jun Liu <yjliu@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790209}