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..b3ebb99eea154d91695242456c49eeb5849d65ff 100644
--- a/third_party/WebKit/Source/platform/graphics/paint/ScrollPaintPropertyNode.h
+++ b/third_party/WebKit/Source/platform/graphics/paint/ScrollPaintPropertyNode.h
@@ -89,6 +89,32 @@ class PLATFORM_EXPORT ScrollPaintPropertyNode
   }
   void clearMainThreadScrollingReasons() { m_mainThreadScrollingReasons = 0; }
 
+#if DCHECK_IS_ON()
+  // The clone function is used by FindPropertiesNeedingUpdate.h for recording
+  // a scroll node before it has been updated, to later detect changes.
+  PassRefPtr<ScrollPaintPropertyNode> clone() const {
+    RefPtr<ScrollPaintPropertyNode> cloned =
+        adoptRef(new ScrollPaintPropertyNode(
+            m_parent, m_scrollOffsetTranslation, m_clip, m_bounds,
+            m_userScrollableHorizontal, m_userScrollableVertical));
+    cloned->addMainThreadScrollingReasons(m_mainThreadScrollingReasons);
+    return cloned;
+  }
+
+  // The equality operator is used by FindPropertiesNeedingUpdate.h for checking
+  // if a scroll node has changed.
+  bool operator==(const ScrollPaintPropertyNode& o) const {
+    // TODO(pdr): Check main thread scrolling reason equality as well. We do
+    // not yet mark nodes as needing a paint property update on main thread
+    // scrolling reason changes.
+    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;
+  }
+#endif
+
  private:
   ScrollPaintPropertyNode(
       PassRefPtr<ScrollPaintPropertyNode> parent,