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/RTCConfiguration+Private.h

@@ -16,13 +16,6 @@ NS_ASSUME_NONNULL_BEGIN
 
 @interface RTCConfiguration ()
 
-/**
- * RTCConfiguration struct representation of this RTCConfiguration. This is
- * needed to pass to the underlying C++ APIs.
- */
-@property(nonatomic, readonly)
-    webrtc::PeerConnectionInterface::RTCConfiguration* nativeConfiguration;
-
 + (webrtc::PeerConnectionInterface::IceTransportsType)
     nativeTransportsTypeForTransportPolicy:(RTCIceTransportPolicy)policy;
 
@@ -55,6 +48,13 @@ NS_ASSUME_NONNULL_BEGIN
 
 + (NSString *)stringForTcpCandidatePolicy:(RTCTcpCandidatePolicy)policy;
 
+/**
+ * RTCConfiguration struct representation of this RTCConfiguration. This is
+ * needed to pass to the underlying C++ APIs.
+ */
+- (webrtc::PeerConnectionInterface::RTCConfiguration *)
+    createNativeConfiguration;
+
 @end
 
 NS_ASSUME_NONNULL_END