js: Use JS_NewObjectWithGivenProto

In mozjs38, JS_NewObject() doesn't take a prototype JSObject anymore;
instead, JS_NewObjectWithGivenProto() is used for that purpose. The
behaviour of JS_NewObject() is changed so that it looks for a default
prototype, which is the same as passing JS::NullPtr() as the proto
parameter in mozjs31.

https://bugzilla.gnome.org/show_bug.cgi?id=776966