[Cast] Software encoder support for varying video frame sizes.

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 ba8c420f8fc6ac8bc1702f9907eab94dd71ee395..92e8547769981ba7294dd4129552e107b91f167c 100644
--- a/media/cast/sender/video_encoder.h
+++ b/media/cast/sender/video_encoder.h
@@ -26,6 +26,10 @@ class VideoEncoder {
 
   virtual ~VideoEncoder() {}
 
+  // Returns true if the size of video frames passed in successive calls to
+  // EncodedVideoFrame() can vary.
+  virtual bool CanEncodeVariedFrameSizes() const = 0;
+
   // If true is returned, the Encoder has accepted the request and will process
   // it asynchronously, running |frame_encoded_callback| on the MAIN
   // CastEnvironment thread with the result.  If false is returned, nothing