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This change is to move the logic of retransmission from AudioSender/
VideoSender to CastTransportSender.

With this change AudioSender and VideoSender will not handle packets any
more. They will simply see frames. Retransmission is now handled by
CastTransportSender.

Added two more public methods for CastTransportSender:
* CancelSendingFrames
  Cancel sending certain frames.
* SendFrameForKickstart
  Sends the last packet of a frame to kick start the session.

cast_simulator shows no regression.
BUG=393042

Review URL: https://codereview.chromium.org/445933002

Cr-Commit-Position: refs/heads/master@{#290094}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@290094 0039d316-1c4b-4281-b951-d872f2087c98