H264: Improve FFmpeg decoder performance by using I420BufferPool.

webrtc/common_video/include/video_frame_buffer.h

 /*
  *  Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 55 matching lines @@
  public:
   I420Buffer(int width, int height);
   I420Buffer(int width, int height, int stride_y, int stride_u, int stride_v);
 
   int width() const override;
   int height() const override;
   const uint8_t* data(PlaneType type) const override;
   // Non-const data access is only allowed if HasOneRef() is true to protect
   // against unexpected overwrites.
   uint8_t* MutableData(PlaneType type) override;
+  // The size of the |data| buffer. The YUV planes are continuous in the buffer.
+  int DataSize() const;

[stefan-webrtc, 2016/02/05 10:18:53]

I'm not sure I like this. Should we instead provide an InitializeData() method
which does the memset? We're now exposing information about how I420Buffer
stores its the buffer internally, which will be bad if we ever decide to change
it to three separate buffers.

[hbos, 2016/02/05 11:23:44]

Fair enough. If that changes, we will no longer be able to rely on this buffer
implementation for ffmpeg (or change so that ffmpeg does not rely on the buffer
being continuous), but at least that's DCHECKed.

   int stride(PlaneType type) const override;
   void* native_handle() const override;
   rtc::scoped_refptr<VideoFrameBuffer> NativeToI420Buffer() override;
 
  protected:
   ~I420Buffer() override;
 
  private:
   const int width_;
   const int height_;
@@ skipping 62 matching lines @@
 // Helper function to crop |buffer| without making a deep copy. May only be used
 // for non-native frames.
 rtc::scoped_refptr<VideoFrameBuffer> ShallowCenterCrop(
     const rtc::scoped_refptr<VideoFrameBuffer>& buffer,
     int cropped_width,
     int cropped_height);
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_COMMON_VIDEO_INCLUDE_VIDEO_FRAME_BUFFER_H_