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

webrtc/modules/audio_mixer/audio_mixer_impl_unittest.cc

 /*
  *  Copyright (c) 2016 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 43 matching lines @@
   MockMixerAudioSource()
       : fake_audio_frame_info_(AudioMixer::Source::AudioFrameInfo::kNormal) {
     ON_CALL(*this, GetAudioFrameWithInfo(_, _))
         .WillByDefault(
             Invoke(this, &MockMixerAudioSource::FakeAudioFrameWithInfo));
   }
 
   MOCK_METHOD2(GetAudioFrameWithInfo,
                AudioFrameInfo(int sample_rate_hz, AudioFrame* audio_frame));
 
-  MOCK_METHOD0(Ssrc, int());
+  MOCK_CONST_METHOD0(PreferredSampleRate, int());
+  MOCK_CONST_METHOD0(Ssrc, int());
 
   AudioFrame* fake_frame() { return &fake_frame_; }
   AudioFrameInfo fake_info() { return fake_audio_frame_info_; }
   void set_fake_info(const AudioFrameInfo audio_frame_info) {
     fake_audio_frame_info_ = audio_frame_info;
   }
 
  private:
   AudioFrame fake_frame_;
   AudioFrameInfo fake_audio_frame_info_;
@@ skipping 287 matching lines @@
       kAudioSources, AudioMixer::Source::AudioFrameInfo::kNormal);
   frame_info[0] = AudioMixer::Source::AudioFrameInfo::kMuted;
   std::fill(frames[0].data_, frames[0].data_ + kDefaultSampleRateHz / 100,
             std::numeric_limits<int16_t>::max());
   std::vector<bool> expected_status(kAudioSources, true);
   expected_status[0] = false;
 
   MixAndCompare(frames, frame_info, expected_status);
 }
 }  // namespace webrtc