Implement lazy block layout prototype behind a runtime flag.

Source/core/rendering/RenderLazyBlock.h

Index: Source/core/rendering/RenderLazyBlock.h
diff --git a/Source/core/rendering/RenderLazyBlock.h b/Source/core/rendering/RenderLazyBlock.h
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+/*
+ * Copyright (C) 2013 Google Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef RenderLazyBlock_h
+#define RenderLazyBlock_h
+
+#include "core/rendering/RenderBlock.h"
+
+namespace WebCore {
+
+class RenderLazyBlock FINAL : public RenderBlock {
+public:
+    RenderLazyBlock(Element*);
+    virtual ~RenderLazyBlock();
+
+    bool isAttached() const { return m_attached; }
+    bool isNested() const;
+
+    void markForNestedLayout()
+    {
+        setNeedsLayout(true);
+        m_isNestedLayout = true;
+    }
+
+    RenderLazyBlock* next() const { return m_next; }
+    RenderLazyBlock* previous() const { return m_previous; }
+
+    RenderBox* firstVisibleChildBox() const { return m_firstVisibleChildBox; }
+    RenderBox* lastVisibleChildBox() const { return m_lastVisibleChildBox; }
+
+private:
+    virtual void paintChildren(PaintInfo& forSelf, const LayoutPoint&, PaintInfo& forChild, bool usePrintRect) OVERRIDE;
+
+    virtual bool isRenderLazyBlock() const OVERRIDE { return true; }
+    virtual void willBeDestroyed() OVERRIDE;
+    virtual void willBeRemovedFromTree() OVERRIDE;
+    virtual void layoutBlock(bool relayoutChildren, LayoutUnit pageLogicalHeight) OVERRIDE;
+
+    void layoutChildren(bool relayoutChildren);
+
+    void attachLazyBlock();
+    void detachLazyBlock();
+
+    RenderLazyBlock* m_next;
+    RenderLazyBlock* m_previous;
+    RenderBox* m_firstVisibleChildBox;
+    RenderBox* m_lastVisibleChildBox;
+    bool m_attached;
+    bool m_isNestedLayout;
+    LayoutRect m_intersectRect;
+    IntRect m_expandedViewportRect;
+};
+
+inline RenderLazyBlock* toRenderLazyBlock(RenderObject* object)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!object || object->isRenderLazyBlock());
+    return static_cast<RenderLazyBlock*>(object);
+}
+
+inline const RenderLazyBlock* toRenderLazyBlock(const RenderObject* object)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!object || object->isRenderLazyBlock());
+    return static_cast<const RenderLazyBlock*>(object);
+}
+
+} // namespace WebCore
+
+#endif // RenderLazyBlock_h