Chromecast: fixes crash in AvPipelineImpl teardown.

(1) Making AvPipelineImpl ref-counted caused it to outlive some of
    its members, resulting in crashes. This is mitigated by introducing
    an explicit Finalize() API for {Audio,Video}PipelineImpl.
(2) "Stop" message was not propagated to the browser-side pipeline at
    all. This is now called in ~CmaRenderer.,,
BUG=internal b/19628205

Review URL:

Cr-Commit-Position: refs/heads/master@{#319453}
5 files changed