[ObjC ARC] Converts ios/chrome/browser/ui/util:util to ARC.

ios/chrome/browser/ui/util/pasteboard_util.mm

 // Copyright 2017 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/chrome/browser/ui/util/pasteboard_util.h"
 
 #import <MobileCoreServices/MobileCoreServices.h>
 #import <UIKit/UIKit.h>
 
 #include "base/strings/sys_string_conversions.h"
 #import "net/base/mac/url_conversions.h"
 #include "url/gurl.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 void StoreURLInPasteboard(const GURL& URL) {
   DCHECK(URL.is_valid());
   NSData* plainText = [base::SysUTF8ToNSString(URL.spec())
       dataUsingEncoding:NSUTF8StringEncoding];
   NSDictionary* copiedItem = @{
     (NSString*)kUTTypeURL : net::NSURLWithGURL(URL),
     (NSString*)kUTTypeUTF8PlainText : plainText,
   };
   [[UIPasteboard generalPasteboard] setItems:@[ copiedItem ]];
 }