[Cast] Halt AudioSender transmission when too many frames are in-flight.

[Cast] Halt AudioSender transmission when too many frames are in-flight.

The approach here is to literally copy most of the logic in VideoSender
over into AudioSender, since VideoSender already solves a number of
transmission/re-transmission issues using heuristics that should work
well for audio.  (This also brings us much closer to being able to merge
AudioSender and VideoSender into one implementation.)

Testing: Confirmed correct halting and recovery behavior between
cast_sender_app and cast_receiver_app (SIGSTOPP'ed and SIGCONT'ed each
to simulate a temporary outage).  Also tested a Chromium sender with a
Chromecast receiver.

BUG=380023
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=278323

[miu, 2014-06-18 20:46:40]

hubbe: PTAL.  For M-37, so need this very soon.  Thanks!

[hubbe, 2014-06-18 21:38:46]

lgtm

[miu, 2014-06-18 21:59:28]

The CQ bit was checked by miu@chromium.org

[commit-bot: I haz the power, 2014-06-18 22:01:22]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/miu@chromium.org/340903003/1

[commit-bot: I haz the power, 2014-06-19 11:22:22]

Change committed as 278323