Unify the task source and task canceller API

I moved away from the `Window` struct all the logic to handle task
sources, into a new struct called `TaskManager`. In a happy world, I'd
be able to just have there two functions, of the types:

```rust
fn task_source<T: TaskSource>(&self, name: TaskSourceName) -> Box<T>
fn task_source_with_canceller<T: TaskSource>(&self, name: TaskSourceName)
  -> (Box<T>, TaskSourceCanceller)
```

And not so much duplicated code. However, because TaskSource can't be a
trait object (because it has generic type parameters), that's not
possible. Instead, I decided to reduce duplicated logic through macros.

For reasons[1], I have to pass both the name of the function with
canceller and the name of the function without, as I'm not able to
concatenate them in the macro itself. I could probably use
`concat_idents` to create both types already defined and reduce the
amount of arguments by one, but that macro is nightly only. At the same
time, not being able to declare macros inside `impl` forces me to pass
`self` as an argument.

All this makes this solution more verbose than it would be ideally. It
does reduce duplication, but it doesn't reduce the size of the file.

[1](https://github.com/rust-lang/rust/issues/29599)

1 files changed, 5 insertions, 4 deletions

diff --git a/components/script/dom/audioscheduledsourcenode.rs b/components/script/dom/audioscheduledsourcenode.rs
index 31527fda3de..24437b88db9 100644
--- a/components/script/dom/audioscheduledsourcenode.rs
+++ b/components/script/dom/audioscheduledsourcenode.rs
@@ -10,7 +10,7 @@ use crate::dom::bindings::inheritance::Castable;
 use crate::dom::bindings::num::Finite;
 use crate::dom::bindings::refcounted::Trusted;
 use crate::dom::bindings::reflector::DomObject;
-use crate::task_source::{TaskSource, TaskSourceName};
+use crate::task_source::TaskSource;
 use dom_struct::dom_struct;
 use servo_media::audio::node::OnEndedCallback;
 use servo_media::audio::node::{AudioNodeInit, AudioNodeMessage, AudioScheduledSourceNodeMessage};
@@ -71,15 +71,16 @@ impl AudioScheduledSourceNodeMethods for AudioScheduledSourceNode {
         let this = Trusted::new(self);
         let global = self.global();
         let window = global.as_window();
-        let task_source = window.dom_manipulation_task_source();
-        let canceller = window.task_canceller(TaskSourceName::DOMManipulation);
+        let (task_source, canceller) = window
+            .task_manager()
+            .dom_manipulation_task_source_with_canceller();
         let callback = OnEndedCallback::new(move || {
             let _ = task_source.queue_with_canceller(
                 task!(ended: move || {
                     let this = this.root();
                     let global = this.global();
                     let window = global.as_window();
-                    window.dom_manipulation_task_source().queue_simple_event(
+                    window.task_manager().dom_manipulation_task_source().queue_simple_event(
                         this.upcast(),
                         atom!("ended"),
                         &window