Add heuristics to skip thumbnail generation when it's unnecessary.

chrome/browser/history/top_sites.h

Index: chrome/browser/history/top_sites.h
diff --git a/chrome/browser/history/top_sites.h b/chrome/browser/history/top_sites.h
index 00d23432e4336897f24a06871c5cbd103b34ef3f..f92b87794dd96e00c3a8e6c0067035158ed1d1d5 100644
--- a/chrome/browser/history/top_sites.h
+++ b/chrome/browser/history/top_sites.h
@@ -79,6 +79,11 @@ class TopSites
   bool GetPageThumbnail(const GURL& url,
                         scoped_refptr<RefCountedBytes>* bytes);
 
+  // Get a thumbnail score for a given page. Returns true iff we have the
+  // thumbnail score.  This may be invoked on any thread. The score will
+  // be copied to |score|.
+  virtual bool GetPageThumbnailScore(const GURL& url, ThumbnailScore* score);
+
   // Invoked from History if migration is needed. If this is invoked it will
   // be before HistoryLoaded is invoked.
   void MigrateFromHistory();
@@ -144,6 +149,19 @@ class TopSites
 
   bool loaded() const { return loaded_; }
 
+  // Returns true if the given URL is known to the top sites service.
+  // This function also returns false if TopSites isn't loaded yet.
+  virtual bool IsKnownURL(const GURL& url);
+
+  // Returns true if the top sites list is full (i.e. we already have the
+  // maximum number of top sites).  This function also returns false if
+  // TopSites isn't loaded yet.
+  virtual bool IsFull();
+
+ protected:
+  // For allowing inheritance.
+  virtual ~TopSites();
+
  private:
   friend class base::RefCountedThreadSafe<TopSites>;
   friend class TopSitesTest;
@@ -177,8 +195,6 @@ class TopSites
     TOP_SITES_LOADED
   };
 
-  ~TopSites();
-
   // Sets the thumbnail without writing to the database. Useful when
   // reading last known top sites from the DB.
   // Returns true if the thumbnail was set, false if the existing one is better.