Closes #6724 (Allows object evaluation in devtools)

The purpose of this is to fix how objects were previously evaluated in
the developer tools.

- Before this, evaluating an object such as the `window` would `panic!`
- After this, evaluating an object such as the `window` outputs `[object
  Window]`

A few things to note:

- This commit contains `unsafe` code.
- This does not contain a test because the developer tools cannot be properly tested until #5971 lands.

components/devtools/actors/object.rs

@@ -0,0 +1,44 @@
+/* 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, ActorRegistry};
+use rustc_serialize::json;
+use std::net::TcpStream;
+
+pub struct ObjectActor {
+    pub name: String,
+    pub uuid: String,
+}
+
+impl Actor for ObjectActor {
+    fn name(&self) -> String {
+        self.name.clone()
+    }
+    fn handle_message(&self,
+                      _: &ActorRegistry,
+                      _: &str,
+                      _: &json::Object,
+                      _: &mut TcpStream) -> Result<bool, ()> {
+        Ok(false)
+    }
+}
+
+impl ObjectActor {
+    pub fn new(registry: &ActorRegistry, uuid: String) -> String {
+        if !registry.script_actor_registered(uuid.clone()) {
+            let name = registry.new_name("object");
+            let actor = ObjectActor {
+                name: name.clone(),
+                uuid: uuid.clone(),
+            };
+
+            registry.register_script_actor(uuid, name.clone());
+            registry.register_later(box actor);
+
+            name
+        } else {
+            registry.script_to_actor(uuid)
+        }
+    }
+}