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This change adds plumbing to allow the LiteVideo optimization to
observe when rebuffer events occur so that all throttling can be
stopped.

A future change will record the even in the user blocklist and
record the event in UKM.

Bug: 1082554
Change-Id: I9109fe221800b355e46c78e80254f75f433f9448
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316849Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Reviewed-by: rajendrant <rajendrant@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Michael Crouse <mcrouse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793232}