Implement a Hit Test Cache.

Source/core/layout/HitTestResult.h

Index: Source/core/layout/HitTestResult.h
diff --git a/Source/core/layout/HitTestResult.h b/Source/core/layout/HitTestResult.h
index 01127a7ecd36ce969ce251bf51e202cc8a8e05e0..3900a468efc81a35c77e3533c5b5da6366e6fb91 100644
--- a/Source/core/layout/HitTestResult.h
+++ b/Source/core/layout/HitTestResult.h
@@ -63,6 +63,8 @@ public:
     HitTestResult& operator=(const HitTestResult&);
     DECLARE_TRACE();
 
+    bool equalForCacheability(const HitTestResult&) const;
+
     // For point-based hit tests, these accessors provide information about the node
     // under the point. For rect-based hit tests they are meaningless (reflect the
     // last candidate node observed in the rect).
@@ -124,6 +126,11 @@ public:
 
     bool isOverLink() const;
 
+    const LayoutRect& validityRect() const { return m_validityRect; }
+    void setValidityRect(const LayoutRect&);
+    void shrinkValidityRect(const LayoutRect&);
+    void intersectValidityRect(const LayoutRect&);
+
     // Return true if the test is a list-based test and we should continue testing.
     bool addNodeToListBasedTestResult(Node*, const HitTestLocation& pointInContainer, const LayoutRect& = LayoutRect());
     bool addNodeToListBasedTestResult(Node*, const HitTestLocation& pointInContainer, const FloatRect&);
@@ -143,6 +150,7 @@ private:
 
     HitTestLocation m_hitTestLocation;
     HitTestRequest m_hitTestRequest;
+    LayoutRect m_validityRect;
 
     RefPtrWillBeMember<Node> m_innerNode;
     RefPtrWillBeMember<Node> m_innerPossiblyPseudoNode;