[ObjC ARC] Converts ios/chrome/common:unit_tests to ARC.

ios/chrome/common/string_util_unittest.mm

 // Copyright 2013 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.
 
 #include "ios/chrome/common/string_util.h"
 
 #import <UIKit/UIKit.h>
 
-#include "base/mac/scoped_nsobject.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "testing/gtest_mac.h"
 #include "testing/platform_test.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace {
 
 TEST(StringUtilTest, ParseStringWithLink) {
   NSArray* const all_test_data = @[
     @{
       @"input" : @"Text without link.",
       @"expected" : @"Text without link.",
       @"link range" : [NSValue valueWithRange:NSMakeRange(NSNotFound, 0)]
     },
     @{
@@ skipping 95 matching lines @@
 
   NSString* leading_calculated =
       SubstringOfWidth(long_string, attributes, leading_width, NO);
   EXPECT_NSEQ(leading, leading_calculated);
 
   NSString* trailing_calculated =
       SubstringOfWidth(long_string, attributes, trailing_width, YES);
   EXPECT_NSEQ(trailing, trailing_calculated);
 }
 }  // namespace