DisplayItemPropertyTreeBuilder: Support (2D) transform and scroll display items.

Source/platform/graphics/paint/ScrollDisplayItem.h

 // Copyright 2014 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 ScrollDisplayItem_h
 #define ScrollDisplayItem_h
 
 #include "platform/geometry/IntSize.h"
 #include "platform/graphics/paint/DisplayItem.h"
 #include "wtf/FastAllocBase.h"
 #include "wtf/PassOwnPtr.h"
 
 namespace blink {
 
 class PLATFORM_EXPORT BeginScrollDisplayItem final : public PairedBeginDisplayItem {
 public:
     BeginScrollDisplayItem(const DisplayItemClientWrapper& client, Type type, const IntSize& currentOffset)
         : PairedBeginDisplayItem(client, type, sizeof(*this))
         , m_currentOffset(currentOffset)
     {
         ASSERT(isScrollType(type));
     }
 
     void replay(GraphicsContext&) override;
     void appendToWebDisplayItemList(WebDisplayItemList*) const override;
 
+    const IntSize& currentOffset() const { return m_currentOffset; }
+
 private:
 #ifndef NDEBUG
     void dumpPropertiesAsDebugString(WTF::StringBuilder&) const final;
 #endif
 
+private:
     const IntSize m_currentOffset;
 };
 
 class PLATFORM_EXPORT EndScrollDisplayItem final : public PairedEndDisplayItem {
 public:
     EndScrollDisplayItem(const DisplayItemClientWrapper& client, Type type)
         : PairedEndDisplayItem(client, type, sizeof(*this))
     {
         ASSERT(isEndScrollType(type));
     }
 
     void replay(GraphicsContext&) override;
     void appendToWebDisplayItemList(WebDisplayItemList*) const override;
 
 private:
 #if ENABLE(ASSERT)
     bool isEndAndPairedWith(DisplayItem::Type otherType) const final { return DisplayItem::isScrollType(otherType); }
 #endif
 };
 
 } // namespace blink
 
 #endif // ScrollDisplayItem_h