Watch time for our purposes is defined as the amount of elapsed media time
for audio+video media. A minimum of 7 seconds of unmuted, foreground media
must be watched to start watch time monitoring. Watch time is checked every
5 seconds from then on and reported to multiple buckets: All, MSE, SRC, EME,
AC, and battery.

Any one of paused, hidden, or muted is sufficient to stop watch time metric
reports. Each of these has a hysteresis where if the state change is undone
within 5 seconds, the watch time will be counted as uninterrupted.

Power events (on/off battery power) have a similar hysteresis, but unlike
the aforementioned properties, will not stop metric collection.

Each seek event will result in a new watch time metric being started and the
old metric finalized as accurately as possible.

BUG=633743
TEST=new unittest.

Review-Url: https://codereview.chromium.org/2160963002
Cr-Commit-Position: refs/heads/master@{#412664}