Implement incremental paint property tree rebuilding

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

Index: third_party/WebKit/Source/platform/graphics/paint/ScrollPaintPropertyNode.h
diff --git a/third_party/WebKit/Source/platform/graphics/paint/ScrollPaintPropertyNode.h b/third_party/WebKit/Source/platform/graphics/paint/ScrollPaintPropertyNode.h
index db2fa0ad73983146c23ba5ea544946d49d7492aa..4f6d7796a1b1809693e2744e3f550a12a69ac78b 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/ScrollPaintPropertyNode.h
+++ b/third_party/WebKit/Source/platform/graphics/paint/ScrollPaintPropertyNode.h
@@ -60,6 +60,22 @@ class PLATFORM_EXPORT ScrollPaintPropertyNode
     m_mainThreadScrollingReasons = 0;
   }
 
+  // TODO(pdr): Correctly clone main thread scrolling reasons.
+  PassRefPtr<ScrollPaintPropertyNode> clone() const {
+    return adoptRef(new ScrollPaintPropertyNode(
+        m_parent, m_scrollOffsetTranslation, m_clip, m_bounds,
+        m_userScrollableHorizontal, m_userScrollableVertical));
+  }
+
+  bool operator==(const ScrollPaintPropertyNode& o) const {
+    return m_parent == o.m_parent &&
+           m_scrollOffsetTranslation == o.m_scrollOffsetTranslation &&
+           m_clip == o.m_clip && m_bounds == o.m_bounds &&
+           m_userScrollableHorizontal == o.m_userScrollableHorizontal &&
+           m_userScrollableVertical == o.m_userScrollableVertical &&
+           m_mainThreadScrollingReasons == o.m_mainThreadScrollingReasons;
+  }
+
   ScrollPaintPropertyNode* parent() const { return m_parent.get(); }
   bool isRoot() const { return !m_parent; }