Update API for Objective-C RTCIceServer

BUG=

Review URL: https://codereview.webrtc.org/1499653003

Cr-Commit-Position: refs/heads/master@{#11000}

webrtc/base/objc/NSString+StdString.h

@@ -0,0 +1,25 @@
+/*
+ *  Copyright 2015 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#import <Foundation/Foundation.h>
+
+#include <string>
+
+NS_ASSUME_NONNULL_BEGIN
+
+@interface NSString (StdString)
+
+@property(nonatomic, readonly) std::string stdString;
+
++ (std::string)stdStringForString:(NSString *)nsString;
+
+@end
+
+NS_ASSUME_NONNULL_END