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

Source/platform/graphics/paint/TransformDisplayItem.h

Index: Source/platform/graphics/paint/TransformDisplayItem.h
diff --git a/Source/platform/graphics/paint/TransformDisplayItem.h b/Source/platform/graphics/paint/TransformDisplayItem.h
index 5d8e89c92f636142e1381cfa112e1bf60c2e428d..d9a20ab0a918c047d36f2a60496b8dce64b61a27 100644
--- a/Source/platform/graphics/paint/TransformDisplayItem.h
+++ b/Source/platform/graphics/paint/TransformDisplayItem.h
@@ -20,11 +20,14 @@ public:
     void replay(GraphicsContext&) override;
     void appendToWebDisplayItemList(WebDisplayItemList*) const override;
 
+    const AffineTransform& transform() const { return m_transform; }
+
 private:
 #ifndef NDEBUG
     void dumpPropertiesAsDebugString(WTF::StringBuilder&) const final;
 #endif
 
+private:
     const AffineTransform m_transform;
 };