H264: Improve FFmpeg decoder performance by using I420BufferPool.

webrtc/common_video/i420_buffer_pool.cc

Index: webrtc/common_video/i420_buffer_pool.cc
diff --git a/webrtc/common_video/i420_buffer_pool.cc b/webrtc/common_video/i420_buffer_pool.cc
index 98daec99f66fcf067ae75ad8998940c0f882f037..c70c14bb6ebbfa2e5e39bcbf6475c692a8917223 100644
--- a/webrtc/common_video/i420_buffer_pool.cc
+++ b/webrtc/common_video/i420_buffer_pool.cc
@@ -53,7 +53,8 @@ class PooledI420Buffer : public webrtc::VideoFrameBuffer {
 
 namespace webrtc {
 
-I420BufferPool::I420BufferPool() {
+I420BufferPool::I420BufferPool(bool zero_initialize)
+    : zero_initialize_(zero_initialize) {
   Release();
 }
 
@@ -83,7 +84,11 @@ rtc::scoped_refptr<VideoFrameBuffer> I420BufferPool::CreateBuffer(int width,
       return new rtc::RefCountedObject<PooledI420Buffer>(buffer);
   }
   // Allocate new buffer.
-  buffers_.push_back(new rtc::RefCountedObject<I420Buffer>(width, height));
+  rtc::scoped_refptr<I420Buffer> buffer = new rtc::RefCountedObject<I420Buffer>(
+      width, height);
+  if (zero_initialize_)
+    memset(buffer->MutableData(kYPlane), 0, buffer->DataSize());

[stefan-webrtc, 2016/02/01 14:02:08]

Is it safe to assume continuous memory here?

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

Yes because it is a pool specifically for I420Buffers which are continuous.
Didn't DCHECK because I420Buffer does not contain |aligned_size| like VideoFrame
does and didn't think it was worth adding.

+  buffers_.push_back(buffer);
   return new rtc::RefCountedObject<PooledI420Buffer>(buffers_.back());
 }