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/Cargo.toml

@@ -31,6 +31,9 @@ path = "../net_traits"
 [dependencies.util]
 path = "../util"
 
+[dependencies.webdriver_server]
+path = "../webdriver_server"
+
 [dependencies.devtools_traits]
 path = "../devtools_traits"
 

components/compositing/constellation.rs

@@ -32,7 +32,7 @@ use std::collections::HashMap;
 use std::io::{self, Write};
 use std::marker::PhantomData;
 use std::mem::replace;
-use std::sync::mpsc::{Receiver, channel};
+use std::sync::mpsc::{Sender, Receiver, channel};
 use url::Url;
 use util::cursor::Cursor;
 use util::geometry::PagePx;
@@ -383,6 +383,10 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
                                                  subpage_id,
                                                  event);
             }
+            ConstellationMsg::GetRootPipeline(resp_chan) => {
+                debug!("constellation got get root pipeline message");
+                self.handle_get_root_pipeline(resp_chan);
+            }
         }
         true
     }
@@ -679,6 +683,14 @@ impl<LTF: LayoutTaskFactory, STF: ScriptTaskFactory> Constellation<LTF, STF> {
         pipeline.trigger_mozbrowser_event(subpage_id, event);
     }
 
+    fn handle_get_root_pipeline(&mut self, resp_chan: Sender<Option<PipelineId>>) {
+        let pipeline_id = self.root_frame_id.map(|frame_id| {
+            let frame = self.frames.get(&frame_id).unwrap();
+            frame.current
+        });
+        resp_chan.send(pipeline_id).unwrap();
+    }
+
     fn add_or_replace_pipeline_in_frame_tree(&mut self, frame_change: FrameChange) {
         let evicted_frames = match frame_change.old_pipeline_id {
             Some(old_pipeline_id) => {

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;