Polishing code to handle certificate generation failure in .mm files.

This is a follow-up to https://codereview.webrtc.org/1965313002/ which
was TBR-landed.

Minor code clean-up/corrections:

Property nativeConfiguration -> - method createNativeConfiguration.
RTCLogWarning -> RTCLogError.
setConfiguration returning NO instead of false.
initWithFactory returning nil instead of nullptr.
Braces around ifs.

Review-Url: https://codereview.webrtc.org/1978233002
Cr-Commit-Position: refs/heads/master@{#12770}

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

@@ -199,9 +199,10 @@ void PeerConnectionDelegateAdapter::OnIceCandidate(
                        delegate:(id<RTCPeerConnectionDelegate>)delegate {
   NSParameterAssert(factory);
   std::unique_ptr<webrtc::PeerConnectionInterface::RTCConfiguration> config(
-      configuration.nativeConfiguration);
-  if (!config)
-    return nullptr;
+      [configuration createNativeConfiguration]);
+  if (!config) {
+    return nil;
+  }
   if (self = [super init]) {
     _observer.reset(new webrtc::PeerConnectionDelegateAdapter(self));
     std::unique_ptr<webrtc::MediaConstraints> nativeConstraints =
@@ -255,9 +256,10 @@ void PeerConnectionDelegateAdapter::OnIceCandidate(
 
 - (BOOL)setConfiguration:(RTCConfiguration *)configuration {
   std::unique_ptr<webrtc::PeerConnectionInterface::RTCConfiguration> config(
-      configuration.nativeConfiguration);
-  if (!config)
-    return false;
+      [configuration createNativeConfiguration]);
+  if (!config) {
+    return NO;
+  }
   return _peerConnection->SetConfiguration(*config);
 }