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Records the maximum number of consecutive NNRs observed during
playback.  "Consecutive" means "separated by at most 60s between
each NNR".  Adds one training example to the newly-introduced
learning task per playback.

Notes:
 NNRs are ignored if we haven't started the smoothness helper yet,
 and i have no idea if this will happen much.  Ideally, not.

 NNR counts are reset when playback is paused, or we adapt, or any
 of the other things that reset the smoothness helper.

Change-Id: I43a36cd053c7e37134dd8d6a6e83a8d3d27f44b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2016197
Commit-Queue: Chrome Cunningham <chcunningham@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735258}