Add a NeededFrequency() method to the AudioMixer::Source interface.

webrtc/api/audio/audio_mixer.h

Index: webrtc/api/audio/audio_mixer.h
diff --git a/webrtc/api/audio/audio_mixer.h b/webrtc/api/audio/audio_mixer.h
index ee39daaf102acb2dc467832da5b118f2edc2eb7b..d9e36e75916bbc598507d158daabcc3509c561d0 100644
--- a/webrtc/api/audio/audio_mixer.h
+++ b/webrtc/api/audio/audio_mixer.h
@@ -44,6 +44,10 @@ class AudioMixer : public rtc::RefCountInterface {
     // A way for a mixer implementation to distinguish participants.
     virtual int Ssrc() = 0;

[aleloi, 2016/10/31 09:31:38]

Unrelated: I think Ssrc should be const as well. The gain from allowing to use
the source in a const setting is greater than the loss of not allowing a
state-changing implementation. And how could it even be that a source needs to
modify state to produce an Ssrc?

[kwiberg-webrtc, 2016/10/31 09:43:20]

Sounds good to me.

 
+    // A way for this source to say that GetAudioFrameWithInfo called
+    // with this frequency or higher will not cause quality loss.
+    virtual int NeededFrequency() const = 0;

[kwiberg-webrtc, 2016/10/31 09:43:19]

So this is the native sample rate of the source? May I suggest
"NativeSampleRate" or "PreferredSampleRate" in that case?

+
     virtual ~Source() {}
   };