Chromecast: CmaMessageFilterHost SetCdm threading change.

Chromecast: CmaMessageFilterHost SetCdm threading change.

* Revert change to do a multi-step thread-hopping destroy.
  This was not a solid idea in the first place.
* Removes UI-thread member function from CmaMessageFilterHost
  (SetCdmOnUiThread) and makes it static.
* Checks validity of MediaPipelineHost reference on CMA thread
  before invoking MediaPipelineHost::SetCdm by maintaining an extra
  map of MediaPipelineHost references exclusively on the CMA thread.

R=lcwu@chromium.org,servolk@chromium.org
BUG=internal b/20188230

Committed: https://crrev.com/06ad6451eae0062a97e01570f1c403ceedc7dfff
Cr-Commit-Position: refs/heads/master@{#325949}

[lcwu1, 2015-04-20 22:43:58]

lgtm

[gunsch, 2015-04-20 22:51:37]

The CQ bit was checked by gunsch@chromium.org

[commit-bot: I haz the power, 2015-04-20 22:52:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1080233003/1

[commit-bot: I haz the power, 2015-04-20 23:25:58]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-04-20 23:25:59]

Try jobs failed on following builders:
  chromium_presubmit on tryserver.chromium.linux (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.linux/builders/chromium_presub...)

[gunsch, 2015-04-20 23:47:14]

The CQ bit was checked by gunsch@chromium.org

[gunsch, 2015-04-20 23:47:14]

The patchset sent to the CQ was uploaded after l-g-t-m from lcwu@chromium.org
Link to the patchset: https://codereview.chromium.org/1080233003/#ps20001
(title: "rename MessageLoopProxy references")

[commit-bot: I haz the power, 2015-04-20 23:48:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1080233003/20001

[commit-bot: I haz the power, 2015-04-21 00:13:52]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-04-21 00:14:45]

Patchset 2 (id:??) landed as
https://crrev.com/06ad6451eae0062a97e01570f1c403ceedc7dfff
Cr-Commit-Position: refs/heads/master@{#325949}