Oilpan: move SVG property hierarchy to the heap.

Source/core/svg/SVGAElement.h

Index: Source/core/svg/SVGAElement.h
diff --git a/Source/core/svg/SVGAElement.h b/Source/core/svg/SVGAElement.h
index 18745cef341f64389184aadecd715b2d14d08874..84e712d6e4fd5b53e9ad6791c6107facf7c75e28 100644
--- a/Source/core/svg/SVGAElement.h
+++ b/Source/core/svg/SVGAElement.h
@@ -31,10 +31,13 @@ namespace blink {
 class SVGAElement final : public SVGGraphicsElement,
                           public SVGURIReference {
     DEFINE_WRAPPERTYPEINFO();
+    WILL_BE_USING_GARBAGE_COLLECTED_MIXIN(SVGAElement);
 public:
     DECLARE_NODE_FACTORY(SVGAElement);
     SVGAnimatedString* svgTarget() { return m_svgTarget.get(); }
 
+    virtual void trace(Visitor*) override;
+
 private:
     explicit SVGAElement(Document&);
 
@@ -60,7 +63,7 @@ private:
 
     virtual bool willRespondToMouseClickEvents() override;
 
-    RefPtr<SVGAnimatedString> m_svgTarget;
+    RefPtrWillBeMember<SVGAnimatedString> m_svgTarget;
     bool m_wasFocusedByMouse;
 };