[cast] Add optional VideoEncoder method to flush frames.

media/cast/sender/video_encoder.h

Index: media/cast/sender/video_encoder.h
diff --git a/media/cast/sender/video_encoder.h b/media/cast/sender/video_encoder.h
index bcb323c12a1fd83fa99cbba0baccf30894e7e0d5..ba8c420f8fc6ac8bc1702f9907eab94dd71ee395 100644
--- a/media/cast/sender/video_encoder.h
+++ b/media/cast/sender/video_encoder.h
@@ -48,6 +48,14 @@ class VideoEncoder {
   // 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. Like
+  // EncodeVideoFrame(), the encoder will process this request asynchronously.
+  virtual void EmitFrames();
 };
 
 }  // namespace cast