[PPAPI] Pepper MediaStream API audio track implementation and example.

ppapi/shared_impl/media_stream_frame_buffer.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef PPAPI_SHARED_IMPL_MEDIA_STREAM_FRAME_BUFFER_H_
 #define PPAPI_SHARED_IMPL_MEDIA_STREAM_FRAME_BUFFER_H_
 
 #include <deque>
 #include <vector>
 
@@ skipping 20 matching lines @@
 //  6. When the frame from the step 5 is consumed, the reader sends the frame
 //     index back to writer via an IPC message.
 //  7. The writer receives the frame index and puts it back to the writer's free
 //     frame queue by calling the writer's |frame_buffer_.Enqueue()|.
 //  8. Go back to step 1.
 class PPAPI_SHARED_EXPORT MediaStreamFrameBuffer {
  public:
   class PPAPI_SHARED_EXPORT Delegate {
    public:
     virtual ~Delegate();
-    // It is called when a new frame is enqueued.
-    virtual void OnNewFrameEnqueued();
+    // It is called before a new frame being enqueued. Return true if the frame
+    // is consumed by |Delegate|, and then the frame will not be added into the
+    // |frame_queue_|.
+    virtual bool OnNewFramePreEnqueued(int32_t index);

[yzshen1, 2014/01/29 21:28:06]

BeforeNewFrameEnqueued() may be more clearer.

That being said, I think the old way is easier to understand: we always enqueue
the frame index, and send a OnNewFrameEqueued() notification.
Why do we need such a change?

[Peng, 2014/01/31 18:54:43]

I though the performance is slightly better by avoiding an extra enqueue and
dequeue. Change it back. Done

   };
 
   // MediaStreamFrameBuffer doesn't own |delegate|, the caller should keep
   // it alive during the MediaStreamFrameBuffer's lifecycle.
   explicit MediaStreamFrameBuffer(Delegate* delegate);
 
   ~MediaStreamFrameBuffer();
 
   int32_t number_of_frames() const { return number_of_frames_; }
 
@@ skipping 31 matching lines @@
 
   // A memory block shared between renderer process and plugin process.
   scoped_ptr<base::SharedMemory> shm_;
 
   DISALLOW_COPY_AND_ASSIGN(MediaStreamFrameBuffer);
 };
 
 }  // namespace ppapi
 
 #endif  // PPAPI_SHAERD_IMPL_MEDIA_STREAM_FRAME_BUFFER_H_