Split global constellation receiver into two (script and compositor)

[jdm]

There are a couple parts to this:

• replace the single receiver with two separate ones
  • make the constellation event loop (Constellation::run) select over the two receivers (see layout_task.rs for a similar model)
  • provide the appropriate sender when creating new script tasks (probably requires returning a second sender from Constellation::start)
• split the constellation_msg::Msg enum into constellation_msg::ScriptMsg and constellation_msg::CompositorMsg enums that only contain the messages that are appropriate for each sender, and update the types of the various senders and receivers involved, and update all users of the newly-typed senders

Code: components/compositing/constellation.rs, components/msg/constellation_msg.rs

[jdm]

After this change it will be easier to tell if there are risks of deadlocks between the constellation and the script tasks, since there won't be a confusing mix of "does script actually use the message type" variants to consider.

[KiChjang]

We might want to add an additional constellation_msg::NetworkMsg for all the flavors of fetch as described by #4576.

I'll take a look at this issue over the weekend and see what I can do.

[KiChjang]

Quick question: will I run into type incompatibilities if I split constellation_msg::Msg into constellation_msg::ScriptMsg and constellation_msg::CompositorMsg while I am receiving messages from the channel? We currently now have the following:

    fn run(&mut self) {
        loop {
            let request = {
                let receiver_from_script = &self.script_receiver;
                let receiver_from_compositor = &self.compositor_receiver;
                select! {
                    msg = receiver_from_script.recv() => msg.unwrap(),
                    msg = receiver_from_compositor.recv() => msg.unwrap()
                }
            };
            if !self.handle_request(request) {
                break;
            }
        }
    }

and receiver_from_script has the type of Receiver<ScriptMsg> and receiver_from_compositor has the type of Receiver<CompositorMsg>.

[jdm]

Yep! This is why we we stuff like http://mxr.mozilla.org/servo/source/components/layout/layout_task.rs#482 in other places.