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

ios/public/provider/chrome/browser/spotlight/spotlight_provider.mm

 // Copyright 2016 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/spotlight/spotlight_provider.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 bool SpotlightProvider::IsSpotlightEnabled() {
   return false;
 }
 
 NSString* SpotlightProvider::GetBookmarkDomain() {
   return nil;
 }
 
 NSString* SpotlightProvider::GetTopSitesDomain() {
   return nil;
 }
 
 NSString* SpotlightProvider::GetActionsDomain() {
   return nil;
 }
 
 NSString* SpotlightProvider::GetCustomAttributeItemID() {
   return nil;
 }
 
 NSArray* SpotlightProvider::GetAdditionalKeywords() {
   return nil;
 }