Provide isAudioEnabled flag to control audio unit.

webrtc/modules/audio_device/ios/voice_processing_audio_unit.h

Index: webrtc/modules/audio_device/ios/voice_processing_audio_unit.h
diff --git a/webrtc/modules/audio_device/ios/voice_processing_audio_unit.h b/webrtc/modules/audio_device/ios/voice_processing_audio_unit.h
index 2f881e77fcdcbb963a95e0a75846cd8bc807090b..73f1f71b5ffb487a32b4d152f15fbc4826f73016 100644
--- a/webrtc/modules/audio_device/ios/voice_processing_audio_unit.h
+++ b/webrtc/modules/audio_device/ios/voice_processing_audio_unit.h
@@ -33,6 +33,9 @@ class VoiceProcessingAudioUnitObserver {
                                     UInt32 num_frames,
                                     AudioBufferList* io_data) = 0;
 
+  // Callback function called when the sample rate changes.
+  virtual void OnSampleRateChange(float sample_rate) = 0;
+
  protected:
   ~VoiceProcessingAudioUnitObserver() {}
 };
@@ -107,6 +110,11 @@ class VoiceProcessingAudioUnit {
                                         UInt32 bus_number,
                                         UInt32 num_frames,
                                         AudioBufferList* io_data);
+  static void OnSampleRateChange(void* in_ref_con,
+                                 AudioUnit audio_unit,
+                                 AudioUnitPropertyID property_id,
+                                 AudioUnitScope scope,
+                                 AudioUnitElement element);
 
   // Notifies observer that samples are needed for playback.
   OSStatus NotifyGetPlayoutData(AudioUnitRenderActionFlags* flags,
@@ -120,6 +128,11 @@ class VoiceProcessingAudioUnit {
                                      UInt32 bus_number,
                                      UInt32 num_frames,
                                      AudioBufferList* io_data);
+  // Notifies observer that sample rate has changed.
+  void NotifySampleRateChange(AudioUnit audio_unit,
+                              AudioUnitPropertyID property_id,
+                              AudioUnitScope scope,
+                              AudioUnitElement element);
 
   // Returns the predetermined format with a specific sample rate. See
   // implementation file for details on format.