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 8cf5b64d82d74631b63bb8b01edd70e608f4b03c..2d983c06fbbe36750a1448c6060b96b0e5354da8 100644
--- a/Source/core/rendering/RenderLayerScrollableArea.h
+++ b/Source/core/rendering/RenderLayerScrollableArea.h
@@ -200,7 +200,7 @@ 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);
@@ -247,8 +247,8 @@ 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;