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

Source/core/svg/SVGTests.h

Index: Source/core/svg/SVGTests.h
diff --git a/Source/core/svg/SVGTests.h b/Source/core/svg/SVGTests.h
index 89cb53aff9b8fc6f7a19c3fb1249bfd76b653d58..88911f9b09f707c9d1986cca35d1c7577494c4a7 100644
--- a/Source/core/svg/SVGTests.h
+++ b/Source/core/svg/SVGTests.h
@@ -49,7 +49,20 @@ public:
     virtual void trace(Visitor*);
 
 protected:
-    explicit SVGTests(SVGElement* contextElement);
+    SVGTests();
+
+    // 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<SVGStaticStringList> m_requiredFeatures;