Remove JSPageInfo::js_compartment.

A js::rust::Compartment is little more than a glorified pointer to the
reflector of a window, so there's no good reason to use it. Instead, this
commit passes a JS<Window> directly when it's necessary.

This also means that we now have to use JS_DefineFunctions rather than
Compartment::define_functions; I believe the former is clearer to the reader
than the extra indirection involved in the latter calling through three
reopsitories.

This commit also simplifies ScriptTask::load to reuse the 'cx' local that is
in scope already, rather than refetching it through js_info.