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This will currently never be exercised, but is required when we support multiple MediaStreamAudioProcessors with different constraints.

TEST=Since we don't support this yet, I have tested with a custom build hard coded to not create |audio_processor_| in MSAP (to simulate custom constraint settings that would cause no audio processing to happen).
BUG=345296

Review URL: https://codereview.chromium.org/594233002

Cr-Commit-Position: refs/heads/master@{#297386}