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..c3703684add763fd2e3504d696b7dccbe3bcb338 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 +149,17 @@ class TopSites
 
   bool loaded() const { return loaded_; }
 
+  // Returns true if the given URL is known to the top sites service.
+  virtual bool IsKnownURL(const GURL& url);

[brettw, 2011/01/26 05:55:20]

Both IsKnownURL and IsFull return false if TopSites isn't loaded yet. This is
probably OK for your use in the thumbnail service, but the comments here should
probably note this behavior (the result may be wrong if called "too early").

[satorux1, 2011/01/26 08:14:46]

Good idea. Done.

+
+  // Returns true if the top sites list is full (i.e. we already have the
+  // maximu number of top sites).

[brettw, 2011/01/26 05:55:20]

spelling: maximum

[satorux1, 2011/01/26 08:14:46]

Done.

+  virtual bool IsFull();
+
+ protected:
+  // For allowing inheritance.
+  virtual ~TopSites();
+
  private:
   friend class base::RefCountedThreadSafe<TopSites>;
   friend class TopSitesTest;
@@ -177,8 +193,6 @@ 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.