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/smil/SMILBoolType.h

@@ -42,7 +42,7 @@ protected:
 
 private:
   // Private constructor: prevent instances beyond my singleton.
-  constexpr SMILBoolType() {}
+  SMILBoolType() {}
 };
 
 } // namespace mozilla