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VideoPlaybackRoughnessReporter tracks moments when each frame was
submitted and when it was displayed. Then series of frames are
split into groups of consecutive frames, where each group takes
about 1/2 second of palyback.

Such groups are also called 'frame windows'. Each windows is
assigned a roughness score that measures how far playback
smoothness was from the ideal playback.
Information about several windows and their roughness score is
aggregated for a couple of playback minutes and then a window with
95-percentile-max-roughness is reported via the provided callback.

Currently roughness is not reported via any server side metrics.
Integration with UMA and UKM coming soon.

Bug: 1042111
Change-Id: I5a12e400f00f91fc0b63d15e88bca5c0ab09d5f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2097951
Commit-Queue: Eugene Zemtsov <eugene@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: Frank Liberato <liberato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750823}