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The buffering controller serves two purposes:
- for URL playback, this will smooth playback when network
  condition is not good enough. This basically avoids continuous
  stuttering in that case.
- for MSE playback, it stops playback when either audio or video
  underruns (which is a special case of the low buffer level state).

BUG=408189

Review URL: https://codereview.chromium.org/509213002

Cr-Commit-Position: refs/heads/master@{#292764}