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

Source/core/layout/ClipRect.h

Index: Source/core/layout/ClipRect.h
diff --git a/Source/core/layout/ClipRect.h b/Source/core/layout/ClipRect.h
index e04690d9cfa25df26a80c78f99fbd9f591fe46e8..347e336ad2110f78a93c2dd920a46ce156ae8a8c 100644
--- a/Source/core/layout/ClipRect.h
+++ b/Source/core/layout/ClipRect.h
@@ -38,11 +38,13 @@ class 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; }
@@ -68,9 +70,14 @@ public:
     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)