Implement incremental paint property tree rebuilding

third_party/WebKit/Source/platform/graphics/paint/ClipPaintPropertyNode.h

Index: third_party/WebKit/Source/platform/graphics/paint/ClipPaintPropertyNode.h
diff --git a/third_party/WebKit/Source/platform/graphics/paint/ClipPaintPropertyNode.h b/third_party/WebKit/Source/platform/graphics/paint/ClipPaintPropertyNode.h
index 28d06ecbcfac07ac921e9b301e252e940c560f06..b29037f82a2a9f6c504441c9a2ed05868a1891d1 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/ClipPaintPropertyNode.h
+++ b/third_party/WebKit/Source/platform/graphics/paint/ClipPaintPropertyNode.h
@@ -43,6 +43,17 @@ class PLATFORM_EXPORT ClipPaintPropertyNode
     m_clipRect = clipRect;
   }
 
+  PassRefPtr<ClipPaintPropertyNode> clone() const {
+    return adoptRef(
+        new ClipPaintPropertyNode(m_parent, m_localTransformSpace, m_clipRect));
+  }
+
+  bool operator==(const ClipPaintPropertyNode& o) const {
+    return m_parent == o.m_parent &&
+           m_localTransformSpace == o.m_localTransformSpace &&
+           m_clipRect == o.m_clipRect;
+  }
+
   const TransformPaintPropertyNode* localTransformSpace() const {
     return m_localTransformSpace.get();
   }