Chromecast: use weak pointers for AvPipelineImpl callbacks on CMA thread.

Without this, there can be circular references keeping AvPipelineImpl
alive after media pipeline destruction.

R=lcwu@chromium.org,halliwell@chromium.org
BUG=internal b/19643195

Review URL: https://codereview.chromium.org/1005703002

Cr-Commit-Position: refs/heads/master@{#320409}
2 files changed