- PlayerTracker interface can be used by any CDM that needs to track media
  players. It can notify media players of new abailable key (to resume playback)
  or the destruction of the CDM.
- PlayerTrackerImpl is a simple implementation of PlayerTracker that can help
  any CDM to implement PlayerTracker interface.
- BrowserCdm is a common CDM interface for browser side CDMs.
- CdmFactory is renamed to BrowserCdmFactory.
- Now BrowserMediaPlayer only needs to call SetCdm() on the player and do not
  need to track CDM <-> player mapping.

TBR=damienv@chromium.org
BUG=373327
TEST=Existing test pages still work.

Review URL: https://codereview.chromium.org/308073004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@274470 0039d316-1c4b-4281-b951-d872f2087c98