Create an Obj-C wrapper of the RtpReceiverObserverInterface.

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCRtpReceiver.h

Index: webrtc/sdk/objc/Framework/Headers/WebRTC/RTCRtpReceiver.h
diff --git a/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCRtpReceiver.h b/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCRtpReceiver.h
index e98a8f3bea1d650b2677dae2d377ff780a997489..6c39a25310da5e9459b24835079e9c529c891b47 100644
--- a/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCRtpReceiver.h
+++ b/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCRtpReceiver.h
@@ -16,6 +16,35 @@
 
 NS_ASSUME_NONNULL_BEGIN
 
+/** Represents the media type of the RtpReceiver. */
+typedef NS_ENUM(NSInteger, RTCRtpMediaType) {
+  RTCRtpMediaTypeAudio,
+  RTCRtpMediaTypeVideo,
+  RTCRtpMediaTypeData,
+};
+
+@class RTCRtpReceiver;
+
+RTC_EXPORT
+@protocol RTCRtpReceiverDelegate <NSObject>
+
+/** Called when the first RTP packet is received.
+ *
+ *  Note: Currently if there are multiple RtpReceivers of the same media type,
+ *  they will all call OnFirstPacketReceived at once.
+ *
+ *  For example, if we create three audio receivers, A/B/C, they will listen to
+ *  the same signal from the underneath network layer. Whenever the first audio packet
+ *  is received, the underneath signal will be fired. All the receivers A/B/C will be
+ *  notified and the callback of the receiver's delegate will be called.
+ *
+ *  The process is the same for video receivers.
+ */
+- (void)rtpReceiver:(RTCRtpReceiver *)rtpReceiver
+    didReceiveFirstPacketForMediaType:(RTCRtpMediaType)mediaType;
+
+@end
+
 RTC_EXPORT
 @protocol RTCRtpReceiver <NSObject>
 
@@ -38,6 +67,9 @@ RTC_EXPORT
  */
 @property(nonatomic, readonly) RTCMediaStreamTrack *track;
 
+/** The delegate for this RtpReceiver. */
+@property(nonatomic, weak) id<RTCRtpReceiverDelegate> delegate;
+
 @end
 
 RTC_EXPORT