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

Source/platform/graphics/paint/SubtreeDisplayItem.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 SubtreeDisplayItem_h
 #define SubtreeDisplayItem_h
 
 #include "platform/geometry/FloatRect.h"
 #include "platform/graphics/paint/DisplayItem.h"
 #include "wtf/Assertions.h"
@@ skipping 44 matching lines @@
     }
 
 private:
     EndSubtreeDisplayItem(const DisplayItemClientWrapper& client, Type type)
         : PairedEndDisplayItem(client, type)
     {
         ASSERT(isEndSubtreeType(type));
     }
 
 #if ENABLE(ASSERT)
-    virtual bool isEndAndPairedWith(const DisplayItem& other) const override final { return other.isBeginSubtree(); }
+    virtual bool isEndAndPairedWith(DisplayItem::Type otherType) const override final { return DisplayItem::isBeginSubtreeType(otherType); }
 #endif
 };
 
 } // namespace blink
 
 #endif // SubtreeDisplayItem_h