Oilpan: move SVG property hierarchy to the heap.

Source/core/svg/SVGSymbolElement.h

Index: Source/core/svg/SVGSymbolElement.h
diff --git a/Source/core/svg/SVGSymbolElement.h b/Source/core/svg/SVGSymbolElement.h
index 32ce25a40fcfbaad2ddc9c3fea96c2c8560f16af..ac943b2d2b3e038a1a723dd8615557e4a4eebcbc 100644
--- a/Source/core/svg/SVGSymbolElement.h
+++ b/Source/core/svg/SVGSymbolElement.h
@@ -30,9 +30,12 @@ namespace blink {
 class SVGSymbolElement final : public SVGElement,
                                public SVGFitToViewBox {
     DEFINE_WRAPPERTYPEINFO();
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(SVGSymbolElement);
 public:
     DECLARE_NODE_FACTORY(SVGSymbolElement);
 
+    virtual void trace(Visitor*) override;
+
 private:
     explicit SVGSymbolElement(Document&);