Issue #1224 - Remove constant expressions from /dom

This excludes DOMProxy handlers in dom bindings because that's intertwined with
codegen and js that needs to be handled together.

dom/svg/SVGMotionSMILType.h

@@ -78,7 +78,7 @@ public:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr SVGMotionSMILType() {}
+  SVGMotionSMILType() {}
 };
 
 } // namespace mozilla