[Cast] Fix shutdown race between CastReceiverImpl/FrameReceiver and frame emit callback run.

media/cast/receiver/frame_receiver.cc

Index: media/cast/receiver/frame_receiver.cc
diff --git a/media/cast/receiver/frame_receiver.cc b/media/cast/receiver/frame_receiver.cc
index 0e794cdb27562096834a3f728f55ed6851c4ffa6..6b5ef86a4c3c4e00598af30da09b4b5b0e5a87eb 100644
--- a/media/cast/receiver/frame_receiver.cc
+++ b/media/cast/receiver/frame_receiver.cc
@@ -256,7 +256,9 @@ void FrameReceiver::EmitAvailableEncodedFrames() {
     }
     cast_environment_->PostTask(CastEnvironment::MAIN,
                                 FROM_HERE,
-                                base::Bind(frame_request_queue_.front(),
+                                base::Bind(&FrameReceiver::EmitOneFrame,
+                                           weak_factory_.GetWeakPtr(),
+                                           frame_request_queue_.front(),
                                            base::Passed(&encoded_frame)));
     frame_request_queue_.pop_front();
   }
@@ -269,6 +271,13 @@ void FrameReceiver::EmitAvailableEncodedFramesAfterWaiting() {
   EmitAvailableEncodedFrames();
 }
 
+void FrameReceiver::EmitOneFrame(const ReceiveEncodedFrameCallback& callback,
+                                 scoped_ptr<EncodedFrame> encoded_frame) const {
+  DCHECK(cast_environment_->CurrentlyOn(CastEnvironment::MAIN));
+  if (!callback.is_null())
+    callback.Run(encoded_frame.Pass());
+}
+
 base::TimeTicks FrameReceiver::GetPlayoutTime(const EncodedFrame& frame) const {
   base::TimeDelta target_playout_delay = target_playout_delay_;
   if (frame.new_playout_delay_ms) {