Move use of DisplayItemList's vector behind an explicit DisplayItems interface.

Source/platform/graphics/paint/Transform3DDisplayItem.h

 // Copyright 2014 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 Transform3DDisplayItem_h
 #define Transform3DDisplayItem_h
 
 #include "platform/graphics/paint/DisplayItem.h"
 #include "platform/transforms/TransformationMatrix.h"
 #include "wtf/Assertions.h"
@@ skipping 38 matching lines @@
         : PairedEndDisplayItem(client, type)
     {
         ASSERT(DisplayItem::isEndTransform3DType(type));
     }
 
     virtual void replay(GraphicsContext&) override;
     virtual void appendToWebDisplayItemList(WebDisplayItemList*) const override;
 
 private:
 #if ENABLE(ASSERT)
-    virtual bool isEndAndPairedWith(const DisplayItem& other) const override final
+    virtual bool isEndAndPairedWith(DisplayItem::Type otherType) const override final
     {
-        return DisplayItem::transform3DTypeToEndTransform3DType(other.type()) == type();
+        return DisplayItem::transform3DTypeToEndTransform3DType(otherType) == type();
     }
 #endif
 };
 
 } // namespace blink
 
 #endif // Transform3DDisplayItem_h