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

webrtc/modules/audio_mixer/audio_mixer_impl_unittest.cc

Index: webrtc/modules/audio_mixer/audio_mixer_impl_unittest.cc
diff --git a/webrtc/modules/audio_mixer/audio_mixer_impl_unittest.cc b/webrtc/modules/audio_mixer/audio_mixer_impl_unittest.cc
index 28976fae8820d1e78cae797ec2ebe9f236fb20e6..e743192d0220672fa85c778ca974f2cd49e37169 100644
--- a/webrtc/modules/audio_mixer/audio_mixer_impl_unittest.cc
+++ b/webrtc/modules/audio_mixer/audio_mixer_impl_unittest.cc
@@ -61,7 +61,8 @@ class MockMixerAudioSource : public AudioMixer::Source {
   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_; }