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

Source/platform/graphics/paint/DisplayItems.cpp

+// 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.
+
+#include "config.h"
+#include "platform/graphics/paint/DisplayItems.h"
+
+namespace blink {
+
+DisplayItems::DisplayItems()
+{
+}
+
+DisplayItems::~DisplayItems()
+{
+}
+
+void DisplayItems::append(PassOwnPtr<DisplayItem> displayItem)
+{
+    m_items.append(displayItem);
+}
+
+void DisplayItems::appendByMoving(const Iterator& it)
+{
+    // Release the underlying OwnPtr to move the display item ownership.
+    ASSERT(!it->isGone());
+    append(it.m_iterator->release());
+}
+
+void DisplayItems::removeLast()
+{
+    m_items.removeLast();
+}
+
+void DisplayItems::clear()
+{
+    m_items.clear();
+}
+
+void DisplayItems::swap(DisplayItems& other)
+{
+    m_items.swap(other.m_items);
+}
+
+void DisplayItems::setGone(const Iterator& it)
+{
+    it.m_iterator->clear();
+}
+
+} // namespace blink