[Cast] VideoFrameFactory interface to vend frames with encoder affinity.

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 6dec102436ed50892f671ca1c5a33ca625e8603c..8d999a21d16109023c38cf3c9637051a6f165f62 100644
--- a/media/cast/sender/external_video_encoder.cc
+++ b/media/cast/sender/external_video_encoder.cc
@@ -15,6 +15,7 @@
 #include "media/cast/cast_defines.h"
 #include "media/cast/logging/logging_defines.h"
 #include "media/cast/net/cast_transport_config.h"
+#include "media/cast/sender/video_frame_factory.h"
 #include "media/video/video_encode_accelerator.h"
 
 namespace media {
@@ -486,5 +487,12 @@ void ExternalVideoEncoder::GenerateKeyFrame() {
 void ExternalVideoEncoder::LatestFrameIdToReference(uint32 /*frame_id*/) {
   // Do nothing not supported.
 }
+
+// Allow the encoder to return a specialized video frame factory. May return
+// null, in which case a default implementation will be used instead.
+scoped_refptr<VideoFrameFactory> ExternalVideoEncoder::GetVideoFrameFactory() {
+  return nullptr;
+}
+
 }  //  namespace cast
 }  //  namespace media