Fixed crash when PCF is destroyed before RTCRtpReceiver in ObjC

Bug: webrtc:9231
Change-Id: Ic532b7661bb8765f0fc2309d2ad530f664ccfd14
Reviewed-on: https://webrtc-review.googlesource.com/87840
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23931}

sdk/objc/Framework/Classes/PeerConnection/RTCRtpReceiver.mm

@@ -36,6 +36,7 @@ void RtpReceiverDelegateAdapter::OnFirstPacketReceived(
 }  // namespace webrtc
 
 @implementation RTCRtpReceiver {
+  RTCPeerConnectionFactory *_factory;
   rtc::scoped_refptr<webrtc::RtpReceiverInterface> _nativeRtpReceiver;
   std::unique_ptr<webrtc::RtpReceiverDelegateAdapter> _observer;
 }
@@ -102,9 +103,11 @@ void RtpReceiverDelegateAdapter::OnFirstPacketReceived(
   return _nativeRtpReceiver;
 }
 
-- (instancetype)initWithNativeRtpReceiver:
-    (rtc::scoped_refptr<webrtc::RtpReceiverInterface>)nativeRtpReceiver {
+- (instancetype)initWithFactory:(RTCPeerConnectionFactory *)factory
+              nativeRtpReceiver:
+                  (rtc::scoped_refptr<webrtc::RtpReceiverInterface>)nativeRtpReceiver {
   if (self = [super init]) {
+    _factory = factory;
     _nativeRtpReceiver = nativeRtpReceiver;
     RTCLogInfo(
         @"RTCRtpReceiver(%p): created receiver: %@", self, self.description);