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

ios/chrome/browser/ntp_tiles/most_visited_sites_observer_bridge.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/ntp_tiles/most_visited_sites_observer_bridge.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace ntp_tiles {
 
 MostVisitedSitesObserverBridge::MostVisitedSitesObserverBridge(
     id<MostVisitedSitesObserving> observer) {
   observer_.reset(observer);
 }
 
 MostVisitedSitesObserverBridge::~MostVisitedSitesObserverBridge() {}
 
 void MostVisitedSitesObserverBridge::OnMostVisitedURLsAvailable(
     const NTPTilesVector& most_visited) {
   [observer_ onMostVisitedURLsAvailable:most_visited];
 }
 
 void MostVisitedSitesObserverBridge::OnIconMadeAvailable(const GURL& site_url) {
   [observer_ onIconMadeAvailable:site_url];
 }
 
 }  // namespace ntp_tiles