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

As described in bug 396413, it was possible for a posted task to be run
after CastReceiverImpl/FrameReceiver were destroyed.  This change fixes
the bug by having FrameReceiver post a cancelable task (i.e., using its
WeakPtr) instead.

BUG=396413

Review URL: https://codereview.chromium.org/491953003

Cr-Commit-Position: refs/heads/master@{#292244}

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) {

media/cast/receiver/frame_receiver.h

@@ -89,6 +89,13 @@ class FrameReceiver : public RtpPayloadFeedback,
   // EmitAvailableEncodedFrames().
   void EmitAvailableEncodedFramesAfterWaiting();
 
+  // Helper that runs |callback|, passing ownership of |encoded_frame| to it.
+  // This method is used by EmitAvailableEncodedFrames() to return to the event
+  // loop, but make sure that FrameReceiver is still alive before the callback
+  // is run.
+  void EmitOneFrame(const ReceiveEncodedFrameCallback& callback,
+                    scoped_ptr<EncodedFrame> encoded_frame) const;
+
   // Computes the playout time for a frame with the given |rtp_timestamp|.
   // Because lip-sync info is refreshed regularly, calling this method with the
   // same argument may return different results.