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

chrome/browser/tab_contents/thumbnail_generator.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_TAB_CONTENTS_THUMBNAIL_GENERATOR_H_
 #define CHROME_BROWSER_TAB_CONTENTS_THUMBNAIL_GENERATOR_H_
 #pragma once
 
 #include <map>
 #include <utility>
 #include <vector>
 
 #include "base/basictypes.h"
 #include "base/callback.h"
 #include "base/linked_ptr.h"
 #include "base/timer.h"
 #include "chrome/browser/renderer_host/backing_store.h"
 #include "chrome/common/notification_observer.h"
 #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;
   // The result of clipping. This can be used to determine if the
   // generated thumbnail is good or not.
   enum ClipResult {
     // The source image is smaller.
     kSourceIsSmaller,
     // Wider than tall, clip horizontally.
     kWiderThanTall,
@@ skipping 78 matching lines @@
   // input bitmap (400x400).
   static SkBitmap GetClippedBitmap(const SkBitmap& bitmap,
                                    int desired_width,
                                    int desired_height,
                                    ClipResult* clip_result);
 
   // Update the thumbnail of the given URL if necessary.
   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,
                                              BackingStore* backing_store);
   virtual void WidgetDidUpdateBackingStore(RenderWidgetHost* widget);
 
   virtual void WidgetDidReceivePaintAtSizeAck(
       RenderWidgetHost* widget,
       int tag,
       const gfx::Size& size);
@@ skipping 36 matching lines @@
   // Map of callback objects by sequence number.
   struct AsyncRequestInfo;
   typedef std::map<int,
                    linked_ptr<AsyncRequestInfo> > ThumbnailCallbackMap;
   ThumbnailCallbackMap callback_map_;
 
   DISALLOW_COPY_AND_ASSIGN(ThumbnailGenerator);
 };
 
 #endif  // CHROME_BROWSER_TAB_CONTENTS_THUMBNAIL_GENERATOR_H_