Re-Compute slow scroll region on frameview visibility change.

third_party/WebKit/Source/core/frame/FrameView.h

 /*
    Copyright (C) 1997 Martin Jones (mjones@kde.org)
              (C) 1998 Waldo Bastian (bastian@kde.org)
              (C) 1998, 1999 Torben Weis (weis@kde.org)
              (C) 1999 Lars Knoll (knoll@kde.org)
              (C) 1999 Antti Koivisto (koivisto@kde.org)
    Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights
    reserved.
 
    This library is free software; you can redistribute it and/or
@@ skipping 818 matching lines @@
   IntRect convertToContainingWidget(const IntRect&) const override;
   IntRect convertFromContainingWidget(const IntRect&) const override;
   IntPoint convertToContainingWidget(const IntPoint&) const override;
   IntPoint convertFromContainingWidget(const IntPoint&) const override;
 
   void updateWidgetGeometriesIfNeeded();
 
   bool wasViewportResized();
   void sendResizeEventIfNeeded();
 
-  void updateScrollableAreaSet();
+  void frameViewVisibilityChanged();
+
+  void updateParentScrollableAreaSet();
 
   void scheduleUpdateWidgetsIfNecessary();
   void updateWidgetsTimerFired(TimerBase*);
   bool updateWidgets();
 
   bool processUrlFragmentHelper(const String&, UrlFragmentBehavior);
   void setFragmentAnchor(Node*);
   void scrollToFragmentAnchor();
   void didScrollTimerFired(TimerBase*);
 
@@ skipping 263 matching lines @@
                   widget.isFrameView());
 DEFINE_TYPE_CASTS(FrameView,
                   ScrollableArea,
                   scrollableArea,
                   scrollableArea->isFrameView(),
                   scrollableArea.isFrameView());
 
 }  // namespace blink
 
 #endif  // FrameView_h