Add a UseCounter for clip CSS or -webkit-clip-path that clips positioned descendants.

Source/core/layout/ClipRect.h

 /*
  * Copyright (C) 2003, 2009, 2012 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 20 matching lines @@
 
 namespace blink {
 
 class HitTestLocation;
 
 class ClipRect {
     WTF_MAKE_FAST_ALLOCATED(ClipRect);
 public:
     ClipRect()
         : m_hasRadius(false)
+        , m_isClippedByClipCss(false)
     { }
 
     ClipRect(const LayoutRect& rect)
         : m_rect(rect)
         , m_hasRadius(false)
+        , m_isClippedByClipCss(false)
     { }
 
     const LayoutRect& rect() const { return m_rect; }
 
     bool hasRadius() const { return m_hasRadius; }
     void setHasRadius(bool hasRadius) { m_hasRadius = hasRadius; }
 
     bool operator==(const ClipRect& other) const { return rect() == other.rect() && hasRadius() == other.hasRadius(); }
     bool operator!=(const ClipRect& other) const { return rect() != other.rect() || hasRadius() != other.hasRadius(); }
     bool operator!=(const LayoutRect& otherRect) const { return rect() != otherRect; }
 
     void intersect(const LayoutRect& other) { m_rect.intersect(other); }
     void intersect(const ClipRect& other)
     {
         m_rect.intersect(other.rect());
         if (other.hasRadius())
             m_hasRadius = true;
     }
     void move(const LayoutSize& size) { m_rect.move(size); }
     void move(const IntSize& size) { m_rect.move(size); }
     void moveBy(const LayoutPoint& point) { m_rect.moveBy(point); }
 
     bool isEmpty() const { return m_rect.isEmpty(); }
     bool intersects(const HitTestLocation&) const;
 
+    // These have no semantic use. They are used for use-counting.
+    bool isClippedByClipCss() const { return m_isClippedByClipCss; }
+    ClipRect& setIsClippedByClipCss() { m_isClippedByClipCss = true; return *this; }
+
 private:
     LayoutRect m_rect;
     bool m_hasRadius;
+    bool m_isClippedByClipCss;
 };
 
 inline ClipRect intersection(const ClipRect& a, const ClipRect& b)
 {
     ClipRect c = a;
     c.intersect(b);
     return c;
 }
 
 } // namespace blink
 
 #endif // ClipRect_h