Fixed gtest_mac.mm after gtest roll to 621.

testing/gtest_mac.mm

 // Copyright (c) 2010 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #import "gtest_mac.h"
 
+#include <string>
+
 #include <gtest/internal/gtest-port.h>
 #include <gtest/internal/gtest-string.h>
 #include <gtest/gtest.h>
 
 #ifdef GTEST_OS_MAC
 
 #import <Foundation/Foundation.h>
 
 namespace testing {
 namespace internal {
 
 // This overloaded version allows comparison between ObjC objects that conform
 // to the NSObject protocol. Used to implement {ASSERT|EXPECT}_EQ().
 GTEST_API_ AssertionResult CmpHelperNSEQ(const char* expected_expression,
                                          const char* actual_expression,
                                          id<NSObject> expected,
                                          id<NSObject> actual) {
   if (expected == actual || [expected isEqual:actual]) {
     return AssertionSuccess();
   }
   return EqFailure(expected_expression,
                    actual_expression,
-                   String([[expected description] UTF8String]),
-                   String([[actual description] UTF8String]),
+                   std::string([[expected description] UTF8String]),
+                   std::string([[actual description] UTF8String]),
                    false);
 }
 
 // This overloaded version allows comparison between ObjC objects that conform
 // to the NSObject protocol. Used to implement {ASSERT|EXPECT}_NE().
 GTEST_API_ AssertionResult CmpHelperNSNE(const char* expected_expression,
                                          const char* actual_expression,
                                          id<NSObject> expected,
                                          id<NSObject> actual) {
   if (expected != actual && ![expected isEqual:actual]) {
     return AssertionSuccess();
   }
   Message msg;
   msg << "Expected: (" << expected_expression << ") != (" << actual_expression
-      << "), actual: " << String([[expected description] UTF8String])
-      << " vs " << String([[actual description] UTF8String]);
+      << "), actual: " << std::string([[expected description] UTF8String])
+      << " vs " << std::string([[actual description] UTF8String]);
   return AssertionFailure(msg);
 }
 
 }  // namespace internal
 }  // namespace testing
 
 #endif  // GTEST_OS_MAC