[Cast] Fix shutdown race between CastReceiverImpl/FrameReceiver and frame emit callback run.

[Cast] Fix shutdown race between CastReceiverImpl/FrameReceiver and frame emit callback run.

As described in bug 396413, it was possible for a posted task to be run
after CastReceiverImpl/FrameReceiver were destroyed.  This change fixes
the bug by having FrameReceiver post a cancelable task (i.e., using its
WeakPtr) instead.

BUG=396413
Committed: https://crrev.com/13d9b1252f8a0d8f0446325f458d23ace590345f
Cr-Commit-Position: refs/heads/master@{#292244}

[miu, 2014-08-27 19:30:27]

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

[miu, 2014-08-27 19:30:27]

hubbe: PTAL.  A simple fix (one of a few) to help resolve the flaky
CastStreamingApiPixelTest.EndToEnd test.

[hubbe, 2014-08-27 20:05:51]

lgtm

[miu, 2014-08-27 21:12:37]

The CQ bit was checked by miu@chromium.org

[commit-bot: I haz the power, 2014-08-27 21:14:04]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/miu@chromium.org/491953003/1

[commit-bot: I haz the power, 2014-08-27 22:15:47]

Committed patchset #1 (id:1) as b79c73f894042b551e5c56197e7d34223bcf4f93

[commit-bot: I haz the power, 2014-09-10 02:54:47]

Patchset 1 (id:??) landed as
https://crrev.com/13d9b1252f8a0d8f0446325f458d23ace590345f
Cr-Commit-Position: refs/heads/master@{#292244}