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

Source/platform/graphics/paint/DrawingDisplayItem.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 DrawingDisplayItem_h
 #define DrawingDisplayItem_h
 
 #include "platform/PlatformExport.h"
 #include "platform/geometry/FloatPoint.h"
 #include "platform/graphics/paint/DisplayItem.h"
@@ skipping 24 matching lines @@
 
     static PassOwnPtr<DrawingDisplayItem> create(const DisplayItemClientWrapper& client, Type type, PassRefPtr<const SkPicture> picture)
     {
         return adoptPtr(new DrawingDisplayItem(client, type, picture));
     }
 
     virtual void replay(GraphicsContext&);
     virtual void appendToWebDisplayItemList(WebDisplayItemList*) const override;
     virtual bool drawsContent() const override;
 
-    PassRefPtr<const SkPicture> picture() const { return m_picture; }
+    const SkPicture* picture() const { return m_picture.get(); }
 
 #if ENABLE(ASSERT)
     void setUnderInvalidationCheckingMode(UnderInvalidationCheckingMode mode) { m_underInvalidationCheckingMode = mode; }
     UnderInvalidationCheckingMode underInvalidationCheckingMode() const { return m_underInvalidationCheckingMode; }
 #endif
 
 private:
 #ifndef NDEBUG
     virtual void dumpPropertiesAsDebugString(WTF::StringBuilder&) const override;
 #endif
 
     RefPtr<const SkPicture> m_picture;
 
 #if ENABLE(ASSERT)
     UnderInvalidationCheckingMode m_underInvalidationCheckingMode;
 #endif
 };
 
 } // namespace blink
 
 #endif // DrawingDisplayItem_h