[cast] Add optional VideoEncoder method to flush frames.

media/cast/sender/video_encoder.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_ENCODER_H_
 #define MEDIA_CAST_SENDER_VIDEO_ENCODER_H_
 
 #include "base/callback.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
@@ skipping 30 matching lines @@
   // Inform the encoder to encode the next frame as a key frame.
   virtual void GenerateKeyFrame() = 0;
 
   // Inform the encoder to only reference frames older or equal to frame_id;
   virtual void LatestFrameIdToReference(uint32 frame_id) = 0;
 
   // Creates a |VideoFrameFactory| object to vend |VideoFrame| object with
   // encoder affinity (defined as offering some sort of performance benefit).
   // This is an optional capability and by default returns null.
   virtual scoped_ptr<VideoFrameFactory> CreateVideoFrameFactory();
+
+  // Instructs the encoder to finish and emit all frames that have been
+  // submitted for encoding. An encoder may hold a certain number of frames for
+  // analysis. Under certain network conditions, particularly when there is
+  // network congestion, it is necessary to flush out of the encoder all
+  // submitted frames so that eventually new frames may be encoded.

[miu, 2015/01/22 01:07:18]

Also in this comment, please add something to the tune of "This is NOT a
synchronous caller-blocking operation."

+  virtual void EmitFrames();
 };
 
 }  // namespace cast
 }  // namespace media
 
 #endif  // MEDIA_CAST_SENDER_VIDEO_ENCODER_H_