Initialize GC mixin bases only when leftmost vtable has been initialized.

Source/core/svg/SVGURIReference.h

Index: Source/core/svg/SVGURIReference.h
diff --git a/Source/core/svg/SVGURIReference.h b/Source/core/svg/SVGURIReference.h
index 842162d0cb88b76d248110f53547bf482fab3f5a..26af7ce2683dca08c712a2aa2e39a9624eca2bc1 100644
--- a/Source/core/svg/SVGURIReference.h
+++ b/Source/core/svg/SVGURIReference.h
@@ -58,7 +58,20 @@ public:
     virtual void trace(Visitor*);
 
 protected:
-    explicit SVGURIReference(SVGElement*);
+    SVGURIReference();
+
+    // FIXME: Oilpan: the construction of this mixin requires heap allocation,
+    // which cannot be safely done with Oilpan until the object that include
+    // this mixin have had its vptr initialized -- so as to be able to
+    // accurately trace the entire object should a GC strike while constructing
+    // the mixin.
+    //
+    // Try to come up with a more natural alternative and solution that doesn't
+    // require hoisting the constructor code for mixins into a separate method
+    // like initialize(). It makes construction of these heap-allocation mixins
+    // safe in the meantime, however.
+    //
+    void initialize(SVGElement*);
 
 private:
     RefPtrWillBeMember<SVGAnimatedString> m_href;