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

media/cast/sender/external_video_encoder.cc

Index: media/cast/sender/external_video_encoder.cc
diff --git a/media/cast/sender/external_video_encoder.cc b/media/cast/sender/external_video_encoder.cc
index 9cc2e8ccbdd40ea43b70b966f020398ee3f011b5..d09dcfe24710458c3e9a65689dd74c227e83e881 100644
--- a/media/cast/sender/external_video_encoder.cc
+++ b/media/cast/sender/external_video_encoder.cc
@@ -347,6 +347,10 @@ ExternalVideoEncoder::ExternalVideoEncoder(
 ExternalVideoEncoder::~ExternalVideoEncoder() {
 }
 
+bool ExternalVideoEncoder::CanEncodeVariedFrameSizes() const {
+  return false;
+}
+
 bool ExternalVideoEncoder::EncodeVideoFrame(
     const scoped_refptr<media::VideoFrame>& video_frame,
     const base::TimeTicks& reference_time,