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

webrtc/api/audio/audio_mixer.h

 /*
  *  Copyright (c) 2011 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 24 matching lines @@
       kError,   // The audio_frame will not be used.
     };
 
     // Overwrites |audio_frame|. The data_ field is overwritten with
     // 10 ms of new audio (either 1 or 2 interleaved channels) at
     // |sample_rate_hz|. All fields in |audio_frame| must be updated.
     virtual AudioFrameInfo GetAudioFrameWithInfo(int sample_rate_hz,
                                                  AudioFrame* audio_frame) = 0;
 
     // A way for a mixer implementation to distinguish participants.
-    virtual int Ssrc() = 0;
+    virtual int Ssrc() const = 0;
+
+    // A way for this source to say that GetAudioFrameWithInfo called
+    // with this sample rate or higher will not cause quality loss.
+    virtual int PreferredSampleRate() const = 0;
 
     virtual ~Source() {}
   };
 
   // Returns true if adding/removing was successful. A source is never
   // added twice and removal is never attempted if a source has not
   // been successfully added to the mixer. Addition and removal can
   // happen on different threads.
   virtual bool AddSource(Source* audio_source) = 0;
   virtual bool RemoveSource(Source* audio_source) = 0;
 
   // Performs mixing by asking registered audio sources for audio. The
   // mixed result is placed in the provided AudioFrame. This method
   // will only be called from a single thread. The rate and channels
   // arguments specify the rate and number of channels of the mix
   // result. All fields in |audio_frame_for_mixing| must be updated.
   virtual void Mix(int sample_rate_hz,
                    size_t number_of_channels,
                    AudioFrame* audio_frame_for_mixing) = 0;
 
  protected:
   // Since the mixer is reference counted, the destructor may be
   // called from any thread.
   ~AudioMixer() override {}
 };
 }  // namespace webrtc
 
 #endif  // WEBRTC_API_AUDIO_AUDIO_MIXER_H_