[Chromecast] Plumbs raw audio through CMA backend.

Raw audio path is used by webaudio and pepper audio. Plumbing it
through the CMA backend allows us to consolidate various audio
output paths with CMA backend, which in turn eliminates the need
for additional shlib interface to output raw audio.

Review URL: https://codereview.chromium.org/1308153005

Cr-Commit-Position: refs/heads/master@{#349873}
12 files changed