Make OnPopularURLsAvailable optional.

components/ntp_tiles/most_visited_sites.h

Index: components/ntp_tiles/most_visited_sites.h
diff --git a/components/ntp_tiles/most_visited_sites.h b/components/ntp_tiles/most_visited_sites.h
index 449c7362528cc2b83d0b6393de6a43dcff1c477a..123b705c6966ce165d53f52d85f134364cf9a5cb 100644
--- a/components/ntp_tiles/most_visited_sites.h
+++ b/components/ntp_tiles/most_visited_sites.h
@@ -105,7 +105,7 @@ class MostVisitedSites : public history::TopSitesObserver,
   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() {}