Expose network thread in RTCPeerConnectionFactory+Private.

Change-Id: I98f352c832425da6c5500f579969025f258c7669
Bug: webrtc:15078
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/300843
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Auto-Submit: Peter Hanspers <peterhanspers@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#39803}

sdk/objc/api/peerconnection/RTCPeerConnectionFactory+Private.h

@@ -29,6 +29,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 @property(nonatomic, readonly) rtc::Thread* signalingThread;
 @property(nonatomic, readonly) rtc::Thread* workerThread;
+@property(nonatomic, readonly) rtc::Thread* networkThread;
 
 @end
 

sdk/objc/api/peerconnection/RTCPeerConnectionFactory.mm

@@ -339,4 +339,8 @@
   return _workerThread.get();
 }
 
+- (rtc::Thread *)networkThread {
+  return _networkThread.get();
+}
+
 @end