Chromecast: fixes crash in AvPipelineImpl teardown.

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.

R=lcwu@chromium.org,erickung@chromium.org,servolk@chromium.org
BUG=internal b/19628205

Committed: https://crrev.com/d8f23a5a8450540db1145ee70591325de570d2cf
Cr-Commit-Position: refs/heads/master@{#319453}

[lcwu1, 2015-03-06 03:42:59]

lgtm

[gunsch, 2015-03-06 15:58:41]

The CQ bit was checked by gunsch@chromium.org

[commit-bot: I haz the power, 2015-03-06 15:59:06]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/987513006/40001

[commit-bot: I haz the power, 2015-03-06 16:18:40]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-03-06 16:19:24]

Patchset 3 (id:??) landed as
https://crrev.com/d8f23a5a8450540db1145ee70591325de570d2cf
Cr-Commit-Position: refs/heads/master@{#319453}