mac: Fix a -Wobjc-redundant-literal-use warning

testing/gtest_mac_unittest.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.
 
 // Note that while this file is in testing/ and tests GTest macros, it is built
 // as part of Chromium's unit_tests target because the project does not build
 // or run GTest's internal test suite.
 
 #import "testing/gtest_mac.h"
 
 #import <Foundation/Foundation.h>
 
 #include "base/mac/scoped_nsautorelease_pool.h"
 #include "testing/gtest/include/gtest/internal/gtest-port.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 TEST(GTestMac, ExpectNSEQ) {
   base::mac::ScopedNSAutoreleasePool pool;
 
   EXPECT_NSEQ(@"a", @"a");
 
   NSString* s1 = [NSString stringWithUTF8String:"a"];
-  NSString* s2 = [NSString stringWithString:@"a"];
+  NSString* s2 = @"a";
   EXPECT_NE(s1, s2);
   EXPECT_NSEQ(s1, s2);
 }
 
 TEST(GTestMac, AssertNSEQ) {
   base::mac::ScopedNSAutoreleasePool pool;
 
   NSNumber* n1 = [NSNumber numberWithInt:42];
   NSNumber* n2 = [NSNumber numberWithInt:42];
   EXPECT_NE(n1, n2);
@@ skipping 14 matching lines @@
 
 TEST(GTestMac, ExpectNSNil) {
   base::mac::ScopedNSAutoreleasePool pool;
 
   EXPECT_NSEQ(nil, nil);
   EXPECT_NSNE(nil, @"a");
   EXPECT_NSNE(@"a", nil);
 
   // TODO(shess): Test that EXPECT_NSNE(nil, nil) fails.
 }