[Cast] Fix RtcpParser to parse negative playout delay deltas in receiver reports.

[Cast] Fix RtcpParser to parse negative playout delay deltas in receiver reports.

Discovered while running cast_simulator when seeing that no frames were
late at the receiver, even under impossible scenarios.

Committed: https://crrev.com/ea92f89a164be551ef31c6ebfb4387ebb3b3ea88
Cr-Commit-Position: refs/heads/master@{#294989}

[miu, 2014-09-16 00:36:14]

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

[hubbe, 2014-09-16 00:37:17]

lgtm

[hubbe, 2014-09-16 00:37:20]

The CQ bit was checked by hubbe@chromium.org

[commit-bot: I haz the power, 2014-09-16 00:48:56]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patchset/575683002/1

[commit-bot: I haz the power, 2014-09-16 02:35:55]

Committed patchset #1 (id:1) as 342f537ce75a5491a21fdea633d8da73a0e56a82

[commit-bot: I haz the power, 2014-09-16 02:38:30]

Patchset 1 (id:??) landed as
https://crrev.com/ea92f89a164be551ef31c6ebfb4387ebb3b3ea88
Cr-Commit-Position: refs/heads/master@{#294989}