Oilpan: fix build after r364654.

third_party/WebKit/Source/core/dom/shadow/SlotAssignment.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 SlotAssignment_h
 #define SlotAssignment_h
 
 #include "platform/heap/Handle.h"
 
 namespace blink {
 
 class HTMLSlotElement;
 class Node;
 class ShadowRoot;
 
-class SlotAssignment final {
+class SlotAssignment final : public NoBaseWillBeGarbageCollected<SlotAssignment> {
 public:
-    SlotAssignment() { }
+    static PassOwnPtrWillBeRawPtr<SlotAssignment> create()
+    {
+        return adoptPtrWillBeNoop(new SlotAssignment);
+    }
+
     HTMLSlotElement* assignedSlotFor(const Node&) const;
     void resolveAssignment(const ShadowRoot&);
 
     DECLARE_TRACE();
 
 private:
+    SlotAssignment() { }
+
     void assign(Node&, HTMLSlotElement&);
     WillBeHeapHashMap<RawPtrWillBeMember<Node>, RawPtrWillBeMember<HTMLSlotElement>> m_assignment;
 };
 
 } // namespace blink
 
 #endif // HTMLSlotAssignment_h