Let PaintAtAck send an int tag instead of the TransportDIB handle.

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 f95e4f9257f270a93d798a0fcbc6831390aa209e..bba224c09bb8b670a05adf057e356e5ecde7e4d7 100644
--- a/chrome/browser/tab_contents/thumbnail_generator.h
+++ b/chrome/browser/tab_contents/thumbnail_generator.h
@@ -80,7 +80,7 @@ class ThumbnailGenerator : public RenderWidgetHostPaintingObserver,
 
   virtual void WidgetDidReceivePaintAtSizeAck(
       RenderWidgetHost* widget,
-      const TransportDIB::Handle& dib_handle,
+      int tag,
       const gfx::Size& size);
 
   // NotificationObserver interface.
@@ -118,9 +118,9 @@ class ThumbnailGenerator : public RenderWidgetHostPaintingObserver,
   // See the setter above.
   bool no_timeout_;
 
-  // Map of callback objects by TransportDIB::Handle.
+  // Map of callback objects by sequence number.
   struct AsyncRequestInfo;
-  typedef std::map<TransportDIB::Handle,
+  typedef std::map<int,
                    linked_ptr<AsyncRequestInfo> > ThumbnailCallbackMap;
   ThumbnailCallbackMap callback_map_;