Adds Analog Gain Control (AGC) to the WebRTC client.

media/audio/mac/audio_low_latency_input_mac.h

Index: media/audio/mac/audio_low_latency_input_mac.h
diff --git a/media/audio/mac/audio_low_latency_input_mac.h b/media/audio/mac/audio_low_latency_input_mac.h
index 52609d615c0fb4ea83311eea9195d886e7238110..794e6310d449c63d98cea7c8827d1773472aa42a 100644
--- a/media/audio/mac/audio_low_latency_input_mac.h
+++ b/media/audio/mac/audio_low_latency_input_mac.h
@@ -38,8 +38,10 @@
 
 #include <AudioUnit/AudioUnit.h>
 
+#include "base/atomicops.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/synchronization/lock.h"
+#include "base/time.h"
 #include "media/audio/audio_io.h"
 #include "media/audio/audio_parameters.h"
 
@@ -64,6 +66,8 @@ class AUAudioInputStream : public AudioInputStream {
   virtual double GetMaxVolume() OVERRIDE;
   virtual void SetVolume(double volume) OVERRIDE;
   virtual double GetVolume() OVERRIDE;
+  virtual void SetAutomaticGainControl(bool enabled) OVERRIDE;
+  virtual bool GetAutomaticGainControl() OVERRIDE;
 
   // Returns the current hardware sample rate for the default input device.
   MEDIA_EXPORT static double HardwareSampleRate();
@@ -140,6 +144,19 @@ class AUAudioInputStream : public AudioInputStream {
   // when querying the volume of each channel.
   int number_of_channels_in_frame_;
 
+  // Keeps track of last time a new volume level was fed to the client in an
+  // OnData() callback.
+  base::Time last_volume_update_time_;
+
+  // Contains last result of internal call to GetVolume(). We save resources
+  // by not quering the capture volume for each callback.
+  double volume_;
+
+  // True when Automatic Gain Control is enabled, false otherwise.
+  // This member is modified on the audio thread and read on the internal
+  // Audio Unit thread in each capture callback.
+  base::subtle::Atomic32 agc_is_enabled_;
+
   DISALLOW_COPY_AND_ASSIGN(AUAudioInputStream);
 };