Implement a basic thread actor

These should represent JavaScript contexts, but not defining them at all
prevent any remote debugging session to be started from Firefox. They shouldn't
be conflated with tab actors.

components/devtools/actors/thread.rs

@@ -0,0 +1,80 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use actor::{Actor, ActorMessageStatus, ActorRegistry};
+use protocol::JsonPacketStream;
+use rustc_serialize::json;
+use std::net::TcpStream;
+
+#[derive(RustcEncodable)]
+struct ThreadAttachedReply {
+    from: String,
+    __type__: String,
+    actor: String,
+    poppedFrames: Vec<PoppedFrameMsg>,
+    why: WhyMsg,
+}
+
+#[derive(RustcEncodable)]
+enum PoppedFrameMsg {}
+
+#[derive(RustcEncodable)]
+struct WhyMsg {
+    __type__: String,
+}
+
+#[derive(RustcEncodable)]
+struct ThreadResumedReply {
+    from: String,
+    __type__: String,
+}
+
+pub struct ThreadActor {
+    name: String,
+}
+
+impl ThreadActor {
+    pub fn new(name: String) -> ThreadActor {
+        ThreadActor {
+            name: name,
+        }
+    }
+}
+
+impl Actor for ThreadActor {
+    fn name(&self) -> String {
+        self.name.clone()
+    }
+
+    fn handle_message(&self,
+                      registry: &ActorRegistry,
+                      msg_type: &str,
+                      _msg: &json::Object,
+                      stream: &mut TcpStream) -> Result<ActorMessageStatus, ()> {
+        Ok(match msg_type {
+            "attach" => {
+                let msg = ThreadAttachedReply {
+                    from: self.name(),
+                    __type__: "paused".to_owned(),
+                    actor: registry.new_name("pause"),
+                    poppedFrames: vec![],
+                    why: WhyMsg { __type__: "attached".to_owned() },
+                };
+                stream.write_json_packet(&msg);
+                ActorMessageStatus::Processed
+            },
+
+            "resume" => {
+                let msg = ThreadResumedReply {
+                    from: self.name(),
+                    __type__: "resumed".to_owned(),
+                };
+                stream.write_json_packet(&msg);
+                ActorMessageStatus::Processed
+            },
+
+            _ => ActorMessageStatus::Ignored,
+        })
+    }
+}