Fixes crash in media::AudioBlockFifo::Push for Apple Thundebolt Displays

Fixes crash in media::AudioBlockFifo::Push for Apple Thundebolt Displays

BUG=434681
TEST=WebRTC demos while adding/removing Thunderbolt cable during an ongoing audio session

Committed: https://crrev.com/622f7624b682917bff90db249d0eb452443b73b4
Cr-Commit-Position: refs/heads/master@{#305004}

[henrika (OOO until Aug 14), 2014-11-20 09:56:32]

henrika@chromium.org changed reviewers:
+ tommi@chromium.org

[henrika (OOO until Aug 14), 2014-11-20 09:58:21]

Tommi, please find a solution to the crash as discussed in the issue. I have
only added xians in CC given the OK on the issue.

[henrika (OOO until Aug 14), 2014-11-20 10:07:48]

Done multiple tests using Apple Thunderbolt Display and can confirm that the
FIFO is dynamically increased by one block when cable is removed and then
inserted again.

[tommi (sloooow) - chröme, 2014-11-20 10:36:59]

lgtm

[henrika (OOO until Aug 14), 2014-11-20 12:06:59]

The CQ bit was checked by henrika@chromium.org

[commit-bot: I haz the power, 2014-11-20 12:07:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/743943002/20001

[commit-bot: I haz the power, 2014-11-20 12:54:16]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2014-11-20 12:54:57]

Patchset 2 (id:??) landed as
https://crrev.com/622f7624b682917bff90db249d0eb452443b73b4
Cr-Commit-Position: refs/heads/master@{#305004}