Add task_source directory

Use DOMManipulationTaskSource whenever possible

components/script/task_source/mod.rs

@@ -0,0 +1,16 @@
+/* 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/. */
+
+pub mod dom_manipulation;
+pub mod file_reading;
+pub mod history_traversal;
+pub mod networking;
+pub mod user_interaction;
+
+use std::result::Result;
+
+pub trait TaskSource<T> {
+    fn queue(&self, msg: T) -> Result<(), ()>;
+    fn clone(&self) -> Box<TaskSource<T> + Send>;
+}