Rename Runnable to Task

The changes are:
 * `*Runnable` -> `*Task`;
 * `RunnableMsg` -> `Task`;
 * `RunnableWrapper` -> `TaskCanceller`;
 * `MainThreadRunnable` -> `MainThreadTask`;
 * `wrap_runnable` -> `wrap_task`;
 * `get_runnable_wrapper` -> `task_canceller`;
 * `handler` -> `run`;
 * `main_thread_handler` -> `run_with_script_thread`.

components/script/task_source/mod.rs

@@ -10,16 +10,19 @@ pub mod performance_timeline;
 pub mod user_interaction;
 
 use dom::globalscope::GlobalScope;
-use script_thread::{Runnable, RunnableWrapper};
+use script_thread::{Task, TaskCanceller};
 use std::result::Result;
 
 pub trait TaskSource {
-    fn queue_with_wrapper<T>(&self,
-                             msg: Box<T>,
-                             wrapper: &RunnableWrapper)
-                             -> Result<(), ()>
-                             where T: Runnable + Send + 'static;
-    fn queue<T: Runnable + Send + 'static>(&self, msg: Box<T>, global: &GlobalScope) -> Result<(), ()> {
-        self.queue_with_wrapper(msg, &global.get_runnable_wrapper())
+    fn queue_with_canceller<T>(
+        &self,
+        msg: Box<T>,
+        canceller: &TaskCanceller,
+    ) -> Result<(), ()>
+    where
+        T: Send + Task + 'static;
+
+    fn queue<T: Task + Send + 'static>(&self, msg: Box<T>, global: &GlobalScope) -> Result<(), ()> {
+        self.queue_with_canceller(msg, &global.task_canceller())
     }
 }