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

Source/platform/graphics/paint/TransformDisplayItem.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 TransformDisplayItem_h
 #define TransformDisplayItem_h
 
 #include "platform/graphics/paint/DisplayItem.h"
 #include "platform/transforms/AffineTransform.h"
 #include "wtf/PassOwnPtr.h"
@@ skipping 32 matching lines @@
     }
 
     EndTransformDisplayItem(const DisplayItemClientWrapper& client)
         : PairedEndDisplayItem(client, EndTransform) { }
 
     virtual void replay(GraphicsContext&) override;
     virtual void appendToWebDisplayItemList(WebDisplayItemList*) const override;
 
 private:
 #if ENABLE(ASSERT)
-    virtual bool isEndAndPairedWith(const DisplayItem& other) const override final { return other.type() == BeginTransform; }
+    virtual bool isEndAndPairedWith(DisplayItem::Type otherType) const override final { return otherType == BeginTransform; }
 #endif
 };
 
 } // namespace blink
 
 #endif // TransformDisplayItem_h