Chromecast: Use default media renderer only for FLAC/Opus for now
Chromecast: Use default media renderer only for FLAC/Opus for now

We've encountered some issue (DCHECK in lock_impl_posix.cc)
when playing MP3 through default media renderer. So this
change will switch back to using CMA renderer for MP3 for
now, while we are investigating further.

BUG=internal b/19625391

Review URL: https://codereview.chromium.org/983203002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#319470}

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