Switch the RootLayerScrolls codepath to use root effect paint properties

third_party/WebKit/Source/platform/graphics/paint/EffectPaintPropertyNode.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 EffectPaintPropertyNode_h
 #define EffectPaintPropertyNode_h
 
 #include "platform/PlatformExport.h"
 #include "wtf/PassRefPtr.h"
 #include "wtf/RefCounted.h"
 #include "wtf/RefPtr.h"
 
 #include <iosfwd>
 
 namespace blink {
 
 // A paint effect created by the opacity css property along with a reference to
-// the parent effect node, or nullptr for the root.
+// the parent effect for inherited effects.
+//
+// The effect tree is rooted at a node with no parent. This root node should
+// not be modified.
+//
 // TODO(pdr): Support more effects than just opacity.
 class PLATFORM_EXPORT EffectPaintPropertyNode : public RefCounted<EffectPaintPropertyNode> {
 public:
     static PassRefPtr<EffectPaintPropertyNode> create(PassRefPtr<const EffectPaintPropertyNode> parent, float opacity)
     {
         return adoptRef(new EffectPaintPropertyNode(std::move(parent), opacity));
     }
 
     void update(PassRefPtr<const EffectPaintPropertyNode> parent, float opacity)
     {
+        DCHECK(!isRoot());
         DCHECK(parent != this);
         m_parent = parent;
         m_opacity = opacity;
     }
 
     float opacity() const { return m_opacity; }
 
     // Parent effect or nullptr if this is the root effect.
     const EffectPaintPropertyNode* parent() const { return m_parent.get(); }
     bool isRoot() const { return !m_parent; }
 
 private:
     EffectPaintPropertyNode(PassRefPtr<const EffectPaintPropertyNode> parent, float opacity)
         : m_parent(parent), m_opacity(opacity) { }
 
     RefPtr<const EffectPaintPropertyNode> m_parent;
     float m_opacity;
 };
 
 // Redeclared here to avoid ODR issues.
 // See platform/testing/PaintPrinters.h.
 void PrintTo(const EffectPaintPropertyNode&, std::ostream*);
 
 } // namespace blink
 
 #endif // EffectPaintPropertyNode_h