H264: Improve FFmpeg decoder performance by using I420BufferPool.

webrtc/common_video/video_frame_buffer.cc

Index: webrtc/common_video/video_frame_buffer.cc
diff --git a/webrtc/common_video/video_frame_buffer.cc b/webrtc/common_video/video_frame_buffer.cc
index 492bc495876a6a21b080252686991c89a2f5840b..3f23def4720d23cb51b855e85714650b43dd73b4 100644
--- a/webrtc/common_video/video_frame_buffer.cc
+++ b/webrtc/common_video/video_frame_buffer.cc
@@ -81,6 +81,10 @@ uint8_t* I420Buffer::MutableData(PlaneType type) {
       static_cast<const VideoFrameBuffer*>(this)->data(type));
 }
 
+size_t I420Buffer::DataSize() const {
+  return stride_y_ * height_ + (stride_u_ + stride_v_) * ((height_ + 1) / 2);

[stefan-webrtc, 2016/02/01 14:02:09]

Shouldn't this be the fully allocated size if you are going to use it for
memsetting?

[hbos, 2016/02/02 16:13:03]

This is the size used fir AlignedMalloc and for memset.

Apparently only the data size has to be memset, not the additional bytes due to
alignment:
https://codereview.chromium.org/383893002/diff/1/media/base/video_frame.cc.

This equation is used in two places in this file so I made a helper function.
Also made it return int because the equations was always evaluated as int.

+}
+
 int I420Buffer::stride(PlaneType type) const {
   switch (type) {
     case kYPlane: