[iOS] Added an initialization method to RTCConfiguration that takes a

native configuration.
Added a getConfiguration getter method to RTCPeerConnection to return
the RTCConfiguration.

BUG=webrtc:7431

Review-Url: https://codereview.webrtc.org/2790833002
Cr-Commit-Position: refs/heads/master@{#17517}

webrtc/sdk/objc/Framework/Classes/RTCPeerConnection.mm

@@ -288,6 +288,12 @@ void PeerConnectionDelegateAdapter::OnIceCandidatesRemoved(
   return _peerConnection->SetConfiguration(*config);
 }
 
+- (RTCConfiguration *)configuration {
+  webrtc::PeerConnectionInterface::RTCConfiguration config =
+    _peerConnection->GetConfiguration();
+  return [[RTCConfiguration alloc] initWithNativeConfiguration:&config];
+}
+
 - (void)close {
   _peerConnection->Close();
 }