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

chrome/browser/history/top_sites_cache.h

 // Copyright (c) 2010 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 CHROME_BROWSER_HISTORY_TOP_SITES_CACHE_H_
 #define CHROME_BROWSER_HISTORY_TOP_SITES_CACHE_H_
 #pragma once
 
 #include <algorithm>
 #include <map>
@@ skipping 27 matching lines @@
 
   // Returns the thumbnail as an Image for the specified url. This adds an entry
   // for |url| if one has not yet been added.
   Images* GetImage(const GURL& url);
 
   // Fetches the thumbnail for the specified url. Returns true if there is a
   // thumbnail for the specified url.
   bool GetPageThumbnail(const GURL& url,
                         scoped_refptr<RefCountedBytes>* bytes);
 
+  // Fetches the thumbnail score for the specified url. Returns true if
+  // there is a thumbnail score for the specified url.
+  bool GetPageThumbnailScore(const GURL& url, ThumbnailScore* score);
+
   // Returns the canonical URL for |url|.
   GURL GetCanonicalURL(const GURL& url);
 
   // Returns true if |url| is known.
   bool IsKnownURL(const GURL& url);
 
   // Returns the index into |top_sites_| for |url|.
   size_t GetURLIndex(const GURL& url);
 
   // Removes any thumbnails that are no longer referenced by the top sites.
@@ skipping 41 matching lines @@
   // Generated from the redirects to and from the most visited pages. See
   // description above typedef for details.
   CanonicalURLs canonical_urls_;
 
   DISALLOW_COPY_AND_ASSIGN(TopSitesCache);
 };
 
 }  // namespace history
 
 #endif  // CHROME_BROWSER_HISTORY_TOP_SITES_CACHE_H_