Make OnPopularURLsAvailable optional.

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 87 matching lines @@
     ICON_COLOR,
     // The item displays a default gray box in place of an icon.
     ICON_DEFAULT,
     NUM_TILE_TYPES,
   };
 
   // The observer to be notified when the list of most visited sites changes.
   class Observer {
    public:
     virtual void OnMostVisitedURLsAvailable(const NTPTilesVector& tiles) = 0;
-    virtual void OnPopularURLsAvailable(const PopularSitesVector& sites) = 0;
+    virtual void OnPopularURLsAvailable(const PopularSitesVector& sites) {}
 
    protected:
     virtual ~Observer() {}
   };
 
   // Construct a MostVisitedSites instance.
   //
   // |prefs|, |top_sites|, and |suggestions| are required and may not be null.
   // |popular_sites| and |supervisor| are optional and if null the associated
   // features will be disabled.
@@ skipping 112 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_