This fixes bug http://code.google.com/p/chromium/issues/detail?id=3907, which...

chrome/browser/render_widget_host.cc

 // Copyright (c) 2006-2008 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.
 
 #include "chrome/browser/render_widget_host.h"
 
 #include "base/gfx/gdi_util.h"
 #include "base/message_loop.h"
 #include "chrome/app/chrome_dll_resource.h"
 #include "chrome/browser/render_process_host.h"
@@ skipping 770 matching lines @@
                                   const gfx::Size& view_size) {
   if (is_hidden_) {
     needs_repainting_on_restore_ = true;
     return;
   }
 
   // TODO(darin): do we need to do something else if our backing store is not
   // the same size as the advertised view?  maybe we just assume there is a
   // full paint on its way?
   BackingStore* backing_store = BackingStoreManager::Lookup(this);
-  if (backing_store && backing_store->size() != view_size)
+  if (!backing_store || (backing_store->size() != view_size))
     return;
 
   RECT damaged_rect, r = clip_rect.ToRECT();
   ScrollDC(backing_store->dc(), dx, dy, NULL, &r, NULL, &damaged_rect);
 
   // TODO(darin): this doesn't work if dx and dy are both non-zero!
   DCHECK(dx == 0 || dy == 0);
 
   // We expect that damaged_rect should equal bitmap_rect.
   DCHECK(gfx::Rect(damaged_rect) == bitmap_rect);
@@ skipping 29 matching lines @@
       &RenderWidgetHost::CheckRendererIsUnresponsive);
 }
 
 void RenderWidgetHost::RendererExited() {
   BackingStoreManager::RemoveBackingStore(this);
 }
 
 void RenderWidgetHost::SystemThemeChanged() {
   Send(new ViewMsg_ThemeChanged(routing_id_));
 }