Record rebuffer count of watch time sessions to UMA.

This allows us to infer a metric equivalent to YouTube's smooth
rate. It records the number of rebuffer events that occur within
a given watch time session under the following keys:

Media.RebuffersCount.{Audio|AudioVideo}.{SRC|MSE,EME}

These are the same categories recorded for MTBR.

BUG=648710, 727862
TEST=new unittest.

Change-Id: Ia115125c3628e8d8436255263387a68f9e225df7
Reviewed-on: https://chromium-review.googlesource.com/522860
Commit-Queue: Dale Curtis <[email protected]>
Reviewed-by: Xiaohan Wang <[email protected]>
Reviewed-by: Ilya Sherman <[email protected]>
Cr-Commit-Position: refs/heads/master@{#477526}
5 files changed