[Cast] Clean-up: Move max_unacked_frames into CongestionControl.

[Cast] Clean-up: Move max_unacked_frames into CongestionControl.

max_unacked_frames isn't being used anywhere except within
CongestionControl now that FrameSender uses media duration to decide
whether to drop frames.

BUG=404813
Committed: https://crrev.com/641adc1265ca1c6ebadddcfcc0319246b65eefde
Cr-Commit-Position: refs/heads/master@{#298551}

[miu, 2014-10-04 01:10:37]

miu@chromium.org changed reviewers:
+ hubbe@chromium.org

[miu, 2014-10-04 01:10:38]

PTAL.

[hubbe, 2014-10-06 20:53:37]

lgtm

[miu, 2014-10-07 18:48:17]

The CQ bit was checked by miu@chromium.org

[commit-bot: I haz the power, 2014-10-07 18:50:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/629493004/20001

[commit-bot: I haz the power, 2014-10-07 20:06:26]

Committed patchset #2 (id:20001) as 2e31d06c953393c423add673021df35618dfca4c

[commit-bot: I haz the power, 2014-10-07 20:07:54]

Patchset 2 (id:??) landed as
https://crrev.com/641adc1265ca1c6ebadddcfcc0319246b65eefde
Cr-Commit-Position: refs/heads/master@{#298551}