Clean up of script timer code.

The code was split into the following two abstractions.
 - OneshotTimers can be used to schedule arbitrary oneshot timers, such
   as XHR-Timeouts.
 - JsTimers (`setTimeout` and `setInterval`) which use OneshotTimers to
   schedule individual callbacks.

With this change the implementation (of JsTimers in particular) is in
much closer alignment with the specification.

components/script/dom/bindings/global.rs

@@ -23,7 +23,7 @@ use net_traits::ResourceThread;
 use profile_traits::mem;
 use script_thread::{CommonScriptMsg, ScriptChan, ScriptPort, ScriptThread};
 use script_traits::{MsDuration, ScriptMsg as ConstellationMsg, TimerEventRequest};
-use timers::{ScheduledCallback, TimerHandle};
+use timers::{OneshotTimerCallback, OneshotTimerHandle};
 use url::Url;
 
 /// A freely-copyable reference to a rooted global object.
@@ -224,9 +224,9 @@ impl<'a> GlobalRef<'a> {
     /// Schedule the given `callback` to be invoked after at least `duration` milliseconds have
     /// passed.
     pub fn schedule_callback(&self,
-                             callback: Box<ScheduledCallback>,
+                             callback: OneshotTimerCallback,
                              duration: MsDuration)
-                             -> TimerHandle {
+                             -> OneshotTimerHandle {
         match *self {
             GlobalRef::Window(window) => window.schedule_callback(callback, duration),
             GlobalRef::Worker(worker) => worker.schedule_callback(callback, duration),
@@ -234,7 +234,7 @@ impl<'a> GlobalRef<'a> {
     }
 
     /// Unschedule a previously-scheduled callback.
-    pub fn unschedule_callback(&self, handle: TimerHandle) {
+    pub fn unschedule_callback(&self, handle: OneshotTimerHandle) {
         match *self {
             GlobalRef::Window(window) => window.unschedule_callback(handle),
             GlobalRef::Worker(worker) => worker.unschedule_callback(handle),