Create an Obj-C wrapper of the RtpReceiverObserverInterface.

Create the RTCRtpReceiverDelegate which is a wrapper over
webrtc::RtpReceiverObserverInterface.
The callback will be called whenever the first rtp packet is received.

Related CL: https://codereview.webrtc.org/2531333003/

BUG=webrtc:6742

Review-Url: https://codereview.webrtc.org/2641923003
Cr-Commit-Position: refs/heads/master@{#16501}

webrtc/sdk/objc/Framework/Classes/RTCRtpReceiver+Private.h

@@ -14,6 +14,20 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+namespace webrtc {
+
+class RtpReceiverDelegateAdapter : public RtpReceiverObserverInterface {
+ public:
+  RtpReceiverDelegateAdapter(RTCRtpReceiver* receiver);
+
+  void OnFirstPacketReceived(cricket::MediaType media_type) override;
+
+ private:
+  __weak RTCRtpReceiver* receiver_;
+};
+
+}  // namespace webrtc
+
 @interface RTCRtpReceiver ()
 
 @property(nonatomic, readonly)
@@ -24,6 +38,8 @@ NS_ASSUME_NONNULL_BEGIN
     (rtc::scoped_refptr<webrtc::RtpReceiverInterface>)nativeRtpReceiver
     NS_DESIGNATED_INITIALIZER;
 
++ (RTCRtpMediaType)mediaTypeForNativeMediaType:(cricket::MediaType)nativeMediaType;
+
 @end
 
 NS_ASSUME_NONNULL_END