[Popular Sites] Move PopularSitesImpl to dedicated file

components/ntp_tiles/most_visited_sites.h

 // 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.
 
 #ifndef COMPONENTS_NTP_TILES_MOST_VISITED_SITES_H_
 #define COMPONENTS_NTP_TILES_MOST_VISITED_SITES_H_
 
 #include <stddef.h>
 
 #include <memory>
@@ skipping 14 matching lines @@
 #include "url/gurl.h"
 
 namespace history {
 class TopSites;
 }
 
 namespace user_prefs {
 class PrefRegistrySyncable;
 }
 
+class PrefService;
+
 namespace ntp_tiles {
 
 class IconCacher;
 
 // Shim interface for SupervisedUserService.
 class MostVisitedSitesSupervisor {
  public:
   struct Whitelist {
     base::string16 title;
     GURL entry_point;
@@ skipping 150 matching lines @@
 
   // For callbacks may be run after destruction.
   base::WeakPtrFactory<MostVisitedSites> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(MostVisitedSites);
 };
 
 }  // namespace ntp_tiles
 
 #endif  // COMPONENTS_NTP_TILES_MOST_VISITED_SITES_H_