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This adds calls to MediaExperimentManager when players start / stop,
and receives callbacks when a player is the only one playing.  Right
now, these callbacks are ignored.

Adds a switch "MediaPowerExperiment" to enable tracking players, in
preparation for the power experiment.

This CL also simplifies MediaExperimentManager.

Change-Id: I02aa61971c7ca59093dcf2d7336709e25b8a70a7
Bug: 1017783
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1879751
Commit-Queue: Frank Liberato <liberato@chromium.org>
Reviewed-by: Chrome Cunningham <chcunningham@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710007}