Update API for Objective-C RTCIceCandidate.

webrtc/base/objc/NSString+StdString.mm

Index: webrtc/base/objc/NSString+StdString.mm
diff --git a/webrtc/base/objc/NSString+StdString.mm b/webrtc/base/objc/NSString+StdString.mm
index 95cbe7aacf00909b5d6f8b72b7fd08c2985cb9f1..3210ff0b65d48e2fd8a0d46380b4cddca07358e9 100644
--- a/webrtc/base/objc/NSString+StdString.mm
+++ b/webrtc/base/objc/NSString+StdString.mm
@@ -22,4 +22,12 @@
                      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