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.,
BUG=internal b/20188230

Review URL:

Cr-Commit-Position: refs/heads/master@{#325949}
4 files changed