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This associates a completed underflow count and total underflow duration
with each segment of watch time reported to UKM.

Generally the completion will always fall within a given segment of
watch time, however it is possible that a completion can happen after a
resolution change or other secondary property update and thus end up in
a subsequent segment.

We impose impose a maximum value of 1 minute for rebuffer durations to
avoid noise from rebuffers which may cross a suspend/resume boundary.

BUG=994021
TEST=new unittests
TBR=meacer,sandersd,holte

Change-Id: I7359d329259ce2cc2c6643c61741cb7ca8473b0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879847Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Dan Sanders <sandersd@chromium.org>
Reviewed-by: Mustafa Emre Acer <meacer@chromium.org>
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709191}