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This exercises the is_cast_audio_only=true and enable_webrtc=false
codepaths which periodically break because they are not built.

Corresponding CL at https://chromium-review.googlesource.com/c/452802/

BUG=667546
TEST=`gn gen out_audio/ --args='is_chromecast=true
is_cast_audio_only=true enable_webrtc=false' && ninja -C out_audio/
cast_shell`

Review-Url: https://codereview.chromium.org/2538883003
Cr-Commit-Position: refs/heads/master@{#456222}