Update API for Objective-C RTCIceCandidate.

webrtc/base/objc/NSString+StdString.mm

 /*
  *  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 "NSString+StdString.h"
 
 @implementation NSString (StdString)
 
 - (std::string)stdString {
   return [NSString stdStringForString:self];
 }
 
 + (std::string)stdStringForString:(NSString *)nsString {
   NSData *charData = [nsString dataUsingEncoding:NSUTF8StringEncoding];
   return std::string(reinterpret_cast<const char *>(charData.bytes),
                      charData.length);
 }
 
++ (NSString *)stringForStdString:(const std::string&)stdString {
+  // std::string may contain null termination character so we construct
+  // using length.
+  return [[NSString alloc] initWithBytes:stdString.data()
+                                  length:stdString.length()
+                                encoding:NSUTF8StringEncoding];
+}
+
 @end