Move the Widget hierarchy to the Oilpan heap.

Source/core/rendering/RenderLayerScrollableArea.h

Index: Source/core/rendering/RenderLayerScrollableArea.h
diff --git a/Source/core/rendering/RenderLayerScrollableArea.h b/Source/core/rendering/RenderLayerScrollableArea.h
index 2954a64701c9fce4dc6d08ca33bede2b3399ab54..e9b5fd1768952f5be4e84b336a63d4f56d5ba944 100644
--- a/Source/core/rendering/RenderLayerScrollableArea.h
+++ b/Source/core/rendering/RenderLayerScrollableArea.h
@@ -198,7 +198,7 @@ private:
     LayoutUnit horizontalScrollbarStart(int minX) const;
     IntSize scrollbarOffset(const Scrollbar*) const;
 
-    PassRefPtr<Scrollbar> createScrollbar(ScrollbarOrientation);
+    PassRefPtrWillBeRawPtr<Scrollbar> createScrollbar(ScrollbarOrientation);
     void destroyScrollbar(ScrollbarOrientation);
 
     void setHasHorizontalScrollbar(bool hasScrollbar);
@@ -245,8 +245,8 @@ private:
     IntPoint m_cachedOverlayScrollbarOffset;
 
     // For areas with overflow, we have a pair of scrollbars.
-    RefPtr<Scrollbar> m_hBar;
-    RefPtr<Scrollbar> m_vBar;
+    RefPtrWillBePersistent<Scrollbar> m_hBar;
+    RefPtrWillBePersistent<Scrollbar> m_vBar;
 
     // Renderers to hold our custom scroll corner.
     RawPtrWillBePersistent<RenderScrollbarPart> m_scrollCorner;