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This class measures the throughput from the time first animation
is started on a compositor, to the time last animation ended.

This is useful to measure the total animation throughput during
a transition that has many animations applied such as login, overview
etc.

Bug: 1150492
Test: covered by unittest

Change-Id: Id0bfec6e57403b7f1ebe5a8068ae095b1412bc7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2550366
Commit-Queue: Mitsuru Oshima <oshima@chromium.org>
Reviewed-by: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833625}