Change Page::m_mainFrame to be a Frame.

Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 1181 matching lines @@
     DisableCompositingQueryAsserts disabler;
 
     if (m_slowRepaintObjectCount > 0)
         return true;
 
     if (contentsInCompositedLayer())
         return false;
 
     // The chromium compositor does not support scrolling a non-composited frame within a composited page through
     // the fast scrolling path, so force slow scrolling in that case.
-    if (m_frame->owner() && !hasCompositedContent() && m_frame->page() && m_frame->page()->mainFrame()->view()->hasCompositedContent())
+    if (m_frame->owner() && !hasCompositedContent() && m_frame->page() && m_frame->page()->mainFrame()->isLocalFrame() && toLocalFrame(m_frame->page()->mainFrame())->view()->hasCompositedContent())
         return true;
 
     if (m_isOverlapped && considerOverlap)
         return true;
 
     if (m_cannotBlitToWindow)
         return true;
 
     if (!m_contentIsOpaque)
         return true;
@@ skipping 2082 matching lines @@
 void FrameView::willRemoveScrollbar(Scrollbar* scrollbar, ScrollbarOrientation orientation)
 {
     ScrollableArea::willRemoveScrollbar(scrollbar, orientation);
     if (AXObjectCache* cache = axObjectCache()) {
         cache->remove(scrollbar);
         cache->handleScrollbarUpdate(this);
     }
 }
 
 } // namespace WebCore