[ObjC ARC] Converts ios/public/provider/chrome/browser/signin:signin to ARC.

ios/public/provider/chrome/browser/signin/chrome_identity.mm

 // Copyright 2015 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 "ios/public/provider/chrome/browser/signin/chrome_identity.h"
 
 #include "base/logging.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 @implementation ChromeIdentity
 
 - (NSString*)userEmail {
   NOTREACHED();
   return nil;
 }
 
 - (NSString*)gaiaID {
   NOTREACHED();
   return nil;
 }
 
 - (NSString*)userFullName {
   NOTREACHED();
   return nil;
 }
 
 - (NSString*)hashedGaiaID {
   NOTREACHED();
   return nil;
 }
 
 @end