Simplify subtree (now subsequence) caching

Source/platform/graphics/paint/SubsequenceDisplayItem.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 SubsequenceDisplayItem_h
+#define SubsequenceDisplayItem_h
+
+#include "platform/geometry/FloatRect.h"
+#include "platform/graphics/paint/DisplayItem.h"
+#include "wtf/Assertions.h"
+
+namespace blink {
+
+class BeginSubsequenceDisplayItem final : public PairedBeginDisplayItem {
+public:
+    BeginSubsequenceDisplayItem(const DisplayItemClientWrapper& client)
+        : PairedBeginDisplayItem(client, BeginSubsequence, sizeof(*this))
+    { }
+};
+
+class EndSubsequenceDisplayItem final : public PairedEndDisplayItem {
+public:
+    EndSubsequenceDisplayItem(const DisplayItemClientWrapper& client)
+        : PairedEndDisplayItem(client, EndSubsequence, sizeof(*this))
+    { }
+
+#if ENABLE(ASSERT)
+    bool isEndAndPairedWith(DisplayItem::Type otherType) const final { return otherType == BeginSubsequence; }
+#endif
+};
+
+} // namespace blink
+
+#endif // SubsequenceDisplayItem_h