Add enough Webdriver support to enable Get().

This is incomplete in several ways:

* It assumes that there's only one constellation (i.e. top level browsing context), ever.
* The session support is very basic indeed (no capabilities)
* Passing channels over channels may not sit well with IPC
* The error handling is mostly missing

components/compositing/lib.rs

@@ -27,6 +27,7 @@ extern crate net_traits;
 #[macro_use]
 extern crate util;
 extern crate gleam;
+extern crate webdriver_server;
 
 extern crate libc;
 extern crate time;