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

chrome/browser/tab_contents/thumbnail_generator.h

Index: chrome/browser/tab_contents/thumbnail_generator.h
diff --git a/chrome/browser/tab_contents/thumbnail_generator.h b/chrome/browser/tab_contents/thumbnail_generator.h
index de57c218ae60bfb0407e569d64e3a3431424e20a..595df3416c9a5e1f012f80f08182e8dbf9bd8d23 100644
--- a/chrome/browser/tab_contents/thumbnail_generator.h
+++ b/chrome/browser/tab_contents/thumbnail_generator.h
@@ -19,10 +19,15 @@
 #include "chrome/common/notification_registrar.h"
 
 class GURL;
+class Profile;
 class RenderWidgetHost;
 class SkBitmap;
 class TabContents;
 
+namespace history {
+class TopSites;
+}
+
 class ThumbnailGenerator : NotificationObserver {
  public:
   typedef Callback1<const SkBitmap&>::Type ThumbnailReadyCallback;
@@ -121,6 +126,11 @@ class ThumbnailGenerator : NotificationObserver {
   static void UpdateThumbnailIfNecessary(TabContents* tab_contents,
                                          const GURL& url);
 
+  // Returns true if we should update the thumbnail of the given URL.
+  static bool ShouldUpdateThumbnail(Profile* profile,
+                                    history::TopSites* top_sites,
+                                    const GURL& url);
+
  private:
   // RenderWidgetHostPaintingObserver implementation.
   virtual void WidgetWillDestroyBackingStore(RenderWidgetHost* widget,