First version of new merge algorithm (not enabled yet)

Source/core/paint/SubtreeRecorder.h

Index: Source/core/paint/SubtreeRecorder.h
diff --git a/Source/core/paint/SubtreeRecorder.h b/Source/core/paint/SubtreeRecorder.h
new file mode 100644
index 0000000000000000000000000000000000000000..9b4ab8efd44034f5f3e41f913b63e57703d50e10
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+++ b/Source/core/paint/SubtreeRecorder.h
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+// 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 SubtreeRecorder_h
+#define SubtreeRecorder_h
+
+#include "core/rendering/PaintPhase.h"
+#include "wtf/PassOwnPtr.h"
+
+namespace blink {
+
+class DisplayItem;
+class DisplayItemList;
+class GraphicsContext;
+class LayoutObject;
+
+// Records subtree information during painting. The recorder's life span covers
+// all painting operations executed during the root of the subtree's paint method
+// for the paint phase.
+class SubtreeRecorder {
+public:
+    SubtreeRecorder(GraphicsContext*, const LayoutObject& root, PaintPhase);
+
+    ~SubtreeRecorder();
+
+    // This method should be called before we actually paint the subtree, then we'll
+    // issue a pair of BeginSubtree/EndSubtree display items to enclose all display
+    // items when we paint the subtree.
+    // Omit this call if we skip painting of the whole subtree (when e.g. the subtree
+    // is out of the viewport, or the whole subtree is not invalidated), then we'll
+    // issue a SubtreeCached display item.
+    void begin();
+
+private:
+    void addDisplayItem(PassOwnPtr<DisplayItem>);
+
+    DisplayItemList* m_displayItemList;
+    const LayoutObject& m_subtreeRoot;
+    const PaintPhase m_paintPhase;
+    bool m_begun;
+};
+
+} // namespace blink
+
+#endif // SubtreeRecorder_h