Refactor: simplify interface of SenderRtcpSession and CastTransport.

media/cast/sender/frame_sender.h

Index: media/cast/sender/frame_sender.h
diff --git a/media/cast/sender/frame_sender.h b/media/cast/sender/frame_sender.h
index 226d3ef961a0a41ec3aed1f95198033dd48c69e7..6f73df38ba8e47cf1a6f7ecdf8f95cb65c49dab5 100644
--- a/media/cast/sender/frame_sender.h
+++ b/media/cast/sender/frame_sender.h
@@ -62,10 +62,28 @@ class FrameSender {
   virtual base::TimeDelta GetInFlightMediaDuration() const = 0;
 
  protected:
+  class RtcpClient : public RtcpObserver {
+   public:
+    explicit RtcpClient(base::WeakPtr<FrameSender> frame_sender);
+    ~RtcpClient() override;
+
+    void OnReceivedCastMessage(const RtcpCastMessage& cast_message) override;
+    void OnReceivedRtt(base::TimeDelta round_trip_time) override;
+    void OnReceivedPli() override;
+
+   private:
+    const base::WeakPtr<FrameSender> frame_sender_;
+  };
   // Schedule and execute periodic sending of RTCP report.
   void ScheduleNextRtcpReport();
   void SendRtcpReport(bool schedule_future_reports);
 
+  // Protected for testability.
+  void OnReceivedCastFeedback(const RtcpCastMessage& cast_feedback);
+
+  // Called when a Pli message is received.
+  void OnReceivedPli();
+
   void OnMeasuredRoundTripTime(base::TimeDelta rtt);
 
   const scoped_refptr<CastEnvironment> cast_environment_;
@@ -89,12 +107,6 @@ class FrameSender {
   void ResendCheck();
   void ResendForKickstart();
 
-  // Protected for testability.
-  void OnReceivedCastFeedback(const RtcpCastMessage& cast_feedback);
-
-  // Called when a Pli message is received.
-  void OnReceivedPli();
-
   // Returns true if too many frames would be in-flight by encoding and sending
   // the next frame having the given |frame_duration|.
   bool ShouldDropNextFrame(base::TimeDelta frame_duration) const;