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

Source/core/svg/SVGFitToViewBox.h

Index: Source/core/svg/SVGFitToViewBox.h
diff --git a/Source/core/svg/SVGFitToViewBox.h b/Source/core/svg/SVGFitToViewBox.h
index 1e0daef3debc526f1f80bc4fb5fc9e3400e466f6..3d21953dcbb47d66c26d8eae5ad94be21ae0ecfa 100644
--- a/Source/core/svg/SVGFitToViewBox.h
+++ b/Source/core/svg/SVGFitToViewBox.h
@@ -73,7 +73,21 @@ public:
     virtual void trace(Visitor*);
 
 protected:
-    explicit SVGFitToViewBox(SVGElement*, PropertyMapPolicy = PropertyMapPolicyAdd);
+    SVGFitToViewBox();
+
+    // 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* contextElement, PropertyMapPolicy = PropertyMapPolicyAdd);
+
     void updateViewBox(const FloatRect&);
     void clearViewBox() { m_viewBox = nullptr; }
     void clearPreserveAspectRatio() { m_preserveAspectRatio = nullptr; }