Simplify subtree (now subsequence) caching

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"
-
-namespace blink {
-
-class BeginSubtreeDisplayItem final : public PairedBeginDisplayItem {
-public:
-    BeginSubtreeDisplayItem(const DisplayItemClientWrapper& client, Type type)
-        : PairedBeginDisplayItem(client, type, sizeof(*this))
-    {
-        ASSERT(isBeginSubtreeType(type));
-    }
-};
-
-class EndSubtreeDisplayItem final : public PairedEndDisplayItem {
-public:
-    EndSubtreeDisplayItem(const DisplayItemClientWrapper& client, Type type)
-        : PairedEndDisplayItem(client, type, sizeof(*this))
-    {
-        ASSERT(isEndSubtreeType(type));
-    }
-
-#if ENABLE(ASSERT)
-    bool isEndAndPairedWith(DisplayItem::Type otherType) const final { return DisplayItem::isBeginSubtreeType(otherType); }
-#endif
-};
-
-} // namespace blink
-
-#endif // SubtreeDisplayItem_h