[cast] Handle frame size changes directly in the VideoToolbox encoder (v2).

media/cast/sender/video_frame_factory.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 MEDIA_CAST_SENDER_VIDEO_FRAME_FACTORY_H_
 #define MEDIA_CAST_SENDER_VIDEO_FRAME_FACTORY_H_
 
 #include "base/memory/scoped_ptr.h"
 #include "base/time/time.h"
 
 namespace gfx {
 class Size;
 }
 
 namespace media {
 
 class VideoFrame;
 
 namespace cast {
 
 // Interface for an object capable of vending video frames. There is no
 // requirement for a |VideoFrameFactory| to be concurrent but it must not be
 // pinned to a specific thread. Indeed, |VideoFrameFactory| implementations are
 // created by cast on the main cast thread then used by unknown client threads
 // via the |VideoFrameInput| interface.
 //
 // Clients are responsible for serialzing access to a |VideoFrameFactory|.
 // Generally speaking, it is expected that a client will be using these objects
 // from a rendering thread or callback (which may execute on different threads
-// but never concurrently with itself). Forcing every implementation to take a
-// lock, even with no contention, is an unnecessary cost, especially on mobile
-// platforms.
+// but never concurrently with itself).
 class VideoFrameFactory {
  public:
   virtual ~VideoFrameFactory() {}
 
   // Creates a |VideoFrame| suitable for input via |InsertRawVideoFrame|. Frames
   // obtained in this manner may provide benefits such memory reuse and affinity
   // with the encoder. The format is guaranteed to be I420 or NV12.
   //
   // This can transiently return null if the encoder is not yet initialized or
-  // is re-initializing.
+  // is re-initializing. Note however that if an encoder does support optimized
+  // frames, its |VideoFrameFactory| must eventually return frames. In practice,
+  // this means that |MaybeCreateFrame| must somehow signal the encoder to
+  // perform whatever initialization is needed to eventually produce frames.
   virtual scoped_refptr<VideoFrame> MaybeCreateFrame(
       const gfx::Size& frame_size, base::TimeDelta timestamp) = 0;
 };
 
 }  // namespace cast
 }  // namespace media
 
 #endif  // MEDIA_CAST_SENDER_VIDEO_FRAME_FACTORY_H_