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, 4 insertions, 5 deletions

diff --git a/components/script/dom/htmlstyleelement.rs b/components/script/dom/htmlstyleelement.rs
index 364640fa169..4c9d12d560a 100644
--- a/components/script/dom/htmlstyleelement.rs
+++ b/components/script/dom/htmlstyleelement.rs
@@ -127,11 +127,10 @@ impl HTMLStyleElement {
         // No subresource loads were triggered, queue load event
         if self.pending_loads.get() == 0 {
             let window = window_from_node(self);
-            window.dom_manipulation_task_source().queue_simple_event(
-                self.upcast(),
-                atom!("load"),
-                &window,
-            );
+            window
+                .task_manager()
+                .dom_manipulation_task_source()
+                .queue_simple_event(self.upcast(), atom!("load"), &window);
         }
 
         self.set_stylesheet(sheet);