Switch RootLayerScrolls to use root transform and scroll properties

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef TransformPaintPropertyNode_h
 #define TransformPaintPropertyNode_h
 
 #include "platform/PlatformExport.h"
 #include "platform/geometry/FloatPoint3D.h"
 #include "platform/transforms/TransformationMatrix.h"
 #include "wtf/PassRefPtr.h"
 #include "wtf/RefCounted.h"
 #include "wtf/RefPtr.h"
 
 #include <iosfwd>
 
 namespace blink {
 
 // A transform created by a css property such as "transform" or "perspective"
-// along with a reference to the parent TransformPaintPropertyNode, or nullptr
-// for the root.
+// along with a reference to the parent TransformPaintPropertyNode.
+//
+// The transform tree is rooted at a node with no parent. This root node should
+// not be modified.
 class PLATFORM_EXPORT TransformPaintPropertyNode : public RefCounted<TransformPaintPropertyNode> {
 public:
     static PassRefPtr<TransformPaintPropertyNode> create(
         PassRefPtr<const TransformPaintPropertyNode> parent,
         const TransformationMatrix& matrix,
         const FloatPoint3D& origin,
         bool flattensInheritedTransform = false,
         unsigned renderingContextID = 0)
     {
         return adoptRef(new TransformPaintPropertyNode(matrix, origin, std::move(parent), flattensInheritedTransform, renderingContextID));
     }
 
     void update(PassRefPtr<const TransformPaintPropertyNode> parent, const TransformationMatrix& matrix, const FloatPoint3D& origin, bool flattensInheritedTransform = false, unsigned renderingContextID = 0)
     {
+        DCHECK(!isRoot());
         DCHECK(parent != this);
         m_parent = parent;
         m_matrix = matrix;
         m_origin = origin;
         m_flattensInheritedTransform = flattensInheritedTransform;
         m_renderingContextID = renderingContextID;
     }
 
     const TransformationMatrix& matrix() const { return m_matrix; }
     const FloatPoint3D& origin() const { return m_origin; }
@@ skipping 35 matching lines @@
     unsigned m_renderingContextID;
 };
 
 // Redeclared here to avoid ODR issues.
 // See platform/testing/PaintPrinters.h.
 void PrintTo(const TransformPaintPropertyNode&, std::ostream*);
 
 } // namespace blink
 
 #endif // TransformPaintPropertyNode_h