Fixed crash when PCF is destroyed before MediaStream in ObjC

Bug: webrtc:9231
Change-Id: I04e76172dd0d5ee5e9040e773e63fd4df0c797ce
Reviewed-on: https://webrtc-review.googlesource.com/84580
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23699}

sdk/objc/Framework/Classes/PeerConnection/RTCPeerConnection+Private.h

@@ -56,6 +56,9 @@ class PeerConnectionDelegateAdapter : public PeerConnectionObserver {
 
 @interface RTCPeerConnection ()
 
+/** The factory used to create this RTCPeerConnection */
+@property(nonatomic, readonly) RTCPeerConnectionFactory *factory;
+
 /** The native PeerConnectionInterface created during construction. */
 @property(nonatomic, readonly) rtc::scoped_refptr<webrtc::PeerConnectionInterface>
     nativePeerConnection;