Switch the Root Layer Scrolls codepath to use root clip paint properties

third_party/WebKit/Source/platform/graphics/paint/ClipPaintPropertyNode.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 ClipPaintPropertyNode_h
 #define ClipPaintPropertyNode_h
 
 #include "platform/PlatformExport.h"
 #include "platform/geometry/FloatRoundedRect.h"
 #include "platform/graphics/paint/TransformPaintPropertyNode.h"
 #include "wtf/PassRefPtr.h"
 #include "wtf/RefCounted.h"
 #include "wtf/RefPtr.h"
 
 #include <iosfwd>
 
 namespace blink {
 
 // A clip rect created by a css property such as "overflow" or "clip".
 // Along with a reference to the transform space the clip rect is based on,
-// and an (optional) parent ClipPaintPropertyNode for inherited clips.
+// and a parent ClipPaintPropertyNode for inherited clips.
+//
+// The clip tree is rooted at a node with no parent. This root node should
+// not be modified.
 class PLATFORM_EXPORT ClipPaintPropertyNode : public RefCounted<ClipPaintPropertyNode> {
 public:
     static PassRefPtr<ClipPaintPropertyNode> create(
         PassRefPtr<const ClipPaintPropertyNode> parent,
         PassRefPtr<const TransformPaintPropertyNode> localTransformSpace,
         const FloatRoundedRect& clipRect)
     {
         return adoptRef(new ClipPaintPropertyNode(std::move(parent), std::move(localTransformSpace), clipRect));
     }
 
     void update(PassRefPtr<const ClipPaintPropertyNode> parent, PassRefPtr<const TransformPaintPropertyNode> localTransformSpace, const FloatRoundedRect& clipRect)
     {
+        DCHECK(!isRoot());
         DCHECK(parent != this);
         m_parent = parent;
         m_localTransformSpace = localTransformSpace;
         m_clipRect = clipRect;
     }
 
     const TransformPaintPropertyNode* localTransformSpace() const { return m_localTransformSpace.get(); }
     const FloatRoundedRect& clipRect() const { return m_clipRect; }
 
     // Reference to inherited clips, or nullptr if this is the only clip.
     const ClipPaintPropertyNode* parent() const { return m_parent.get(); }
     bool isRoot() const { return !m_parent; }
 
 private:
     ClipPaintPropertyNode(PassRefPtr<const ClipPaintPropertyNode> parent, PassRefPtr<const TransformPaintPropertyNode> localTransformSpace, const FloatRoundedRect& clipRect)
         : m_parent(parent), m_localTransformSpace(localTransformSpace), m_clipRect(clipRect) { }
 
     RefPtr<const ClipPaintPropertyNode> m_parent;
     RefPtr<const TransformPaintPropertyNode> m_localTransformSpace;
     FloatRoundedRect m_clipRect;
 };
 
 // Redeclared here to avoid ODR issues.
 // See platform/testing/PaintPrinters.h.
 void PrintTo(const ClipPaintPropertyNode&, std::ostream*);
 
 } // namespace blink
 
 #endif // ClipPaintPropertyNode_h