Allow certain SP recorder classes to defer their "begin" operation.

Source/core/paint/ScrollRecorder.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 ScrollRecorder_h
 #define ScrollRecorder_h
 
 #include "core/CoreExport.h"
 #include "core/paint/PaintPhase.h"
 #include "platform/geometry/IntSize.h"
 #include "platform/graphics/paint/DisplayItem.h"
+#include "platform/heap/GarbageCollected.h"
 #include "wtf/FastAllocBase.h"
 
 namespace blink {
 
 class GraphicsContext;
 
 // Emits display items which represent a region which is scrollable, so that it
 // can be translated by the scroll offset.
 class CORE_EXPORT ScrollRecorder {
     WTF_MAKE_FAST_ALLOCATED(ScrollRecorder);
 public:
+    ScrollRecorder(GraphicsContext&, const DisplayItemClientWrapper&, PaintPhase);
     ScrollRecorder(GraphicsContext&, const DisplayItemClientWrapper&, PaintPhase, const IntSize& currentOffset);
     ~ScrollRecorder();
+    void begin(const IntSize& currentOffset);
 private:
     DisplayItemClientWrapper m_client;
     DisplayItem::Type m_beginItemType;
     GraphicsContext& m_context;
+    bool m_engaged;
 };
 
 } // namespace blink
 
 #endif // ScrollRecorder_h