Use multiple shared memory buffers cyclically for audio capture.

media/audio/audio_device_thread.h

Index: media/audio/audio_device_thread.h
===================================================================
--- media/audio/audio_device_thread.h	(revision 186364)
+++ media/audio/audio_device_thread.h	(working copy)
@@ -38,7 +38,8 @@
    public:
     Callback(const AudioParameters& audio_parameters,
              base::SharedMemoryHandle memory,
-             int memory_length);
+             int memory_length,
+             int total_segments);
     virtual ~Callback();
 
     // One time initialization for the callback object on the audio thread.
@@ -49,7 +50,8 @@
     virtual void MapSharedMemory() = 0;
 
     // Called whenever we receive notifications about pending data.
-    virtual void Process(int pending_data) = 0;
+    // The |index| states which shared memory buffer segment is used.
+    virtual void Process(int pending_data, int index) = 0;
 
    protected:
     // Protected so that derived classes can access directly.
@@ -61,12 +63,14 @@
 
     base::SharedMemory shared_memory_;
     const int memory_length_;
+    const int total_segments_;
+    int segment_length_;
 
    private:
     DISALLOW_COPY_AND_ASSIGN(Callback);
   };
 
-  AudioDeviceThread();
+  AudioDeviceThread(bool need_read_index);
   ~AudioDeviceThread();
 
   // Starts the audio thread. The thread must not already be running.
@@ -98,6 +102,9 @@
   //    reliable initialization.
   class Thread;
 
+  // Flag indicating whether need to read index from sync socket.
+  bool need_read_index_;
+
   base::Lock thread_lock_;
   scoped_refptr<AudioDeviceThread::Thread> thread_;