Add JSCLASS_DOM_GLOBAL to DOM global classes, and assert it is used.

It seems unlikely that this will ever be hit, but potentially hard to
figure out if it ever is hit.

src/components/script/dom/bindings/utils.rs

@@ -207,7 +207,7 @@ pub struct DOMJSClass {
 
 pub fn GetProtoOrIfaceArray(global: *JSObject) -> **JSObject {
     unsafe {
-        /*assert ((*JS_GetClass(global)).flags & JSCLASS_DOM_GLOBAL) != 0;*/
+        assert!(((*JS_GetClass(global)).flags & JSCLASS_DOM_GLOBAL) != 0);
         JS_GetReservedSlot(global, DOM_PROTOTYPE_SLOT).to_private() as **JSObject
     }
 }