Unassociating MediaSession from media players (in blink & content)

Since media elements no long joins MediaSession, we should
decouple them.

This is part of a greater effort of making MediaSession API
implementation in sync with the spec.
https://docs.google.com/a/google.com/document/d/1RNaWRpEDCGfDI5HEmMYxqA2QaD2eAOuhTlmuKoWMA8Y/edit?usp=sharing

BUG=646906

Review-Url: https://codereview.chromium.org/2338223003
Cr-Commit-Position: refs/heads/master@{#420418}
30 files changed