[Cast] Clean-up RtpCastHeader and RtpParser, removing the last WebRTC dependency.

media/cast/rtp_receiver/rtp_receiver.h

Index: media/cast/rtp_receiver/rtp_receiver.h
diff --git a/media/cast/rtp_receiver/rtp_receiver.h b/media/cast/rtp_receiver/rtp_receiver.h
index 421d124b4452a6c3f46ae8f5ebdf2e27aaebe717..b4fe99d30eb232623c55506ef12bedb28f21b040 100644
--- a/media/cast/rtp_receiver/rtp_receiver.h
+++ b/media/cast/rtp_receiver/rtp_receiver.h
@@ -32,6 +32,12 @@ class RtpReceiver : public RtpParser {
     return &stats_;
   }
 
+ protected:
+  // Subclasses implement this to consume and process deserialized packets.
+  virtual void OnReceivedPayloadData(const uint8* payload_data,
+                                     size_t payload_size,
+                                     const RtpCastHeader& rtp_header) = 0;
+
  private:
   ReceiverStats stats_;