Add a flag to chrome://flags for experimenting with MediaSession on desktop

This CL adds a flag for experimenting with MediaSession on desktop to chrome://flags. kFlashJoinsMediaSession is merged into
kEnableDefaultMediaSession to make chrome://flags simpler.

BUG=619084

Review-Url: https://chromiumcodereview.appspot.com/2428873002
Cr-Commit-Position: refs/heads/master@{#426219}
7 files changed