[cast] Add optional VideoEncoder method to flush frames.

media/cast/sender/video_encoder.cc

 // 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.
 
 #include "media/cast/sender/video_encoder.h"
 #include "media/cast/sender/video_frame_factory.h"
 
 namespace media {
 namespace cast {
 
 scoped_ptr<VideoFrameFactory> VideoEncoder::CreateVideoFrameFactory() {
   return nullptr;
 }
 
+void VideoEncoder::EmitFrames() {
+}
+
 }  // namespace cast
 }  // namespace media