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, 11 insertions, 9 deletions

diff --git a/components/script/dom/htmlscriptelement.rs b/components/script/dom/htmlscriptelement.rs
index 47a4950b3d6..d93b5357e24 100644
--- a/components/script/dom/htmlscriptelement.rs
+++ b/components/script/dom/htmlscriptelement.rs
@@ -25,7 +25,6 @@ use crate::dom::node::{document_from_node, window_from_node};
 use crate::dom::node::{ChildrenMutation, CloneChildrenFlag, Node};
 use crate::dom::virtualmethods::VirtualMethods;
 use crate::network_listener::{NetworkListener, PreInvoke};
-use crate::task_source::TaskSourceName;
 use dom_struct::dom_struct;
 use encoding_rs::Encoding;
 use html5ever::{LocalName, Prefix};
@@ -294,10 +293,14 @@ fn fetch_a_classic_script(
     }));
 
     let (action_sender, action_receiver) = ipc::channel().unwrap();
+    let (task_source, canceller) = doc
+        .window()
+        .task_manager()
+        .networking_task_source_with_canceller();
     let listener = NetworkListener {
-        context: context,
-        task_source: doc.window().networking_task_source(),
-        canceller: Some(doc.window().task_canceller(TaskSourceName::Networking)),
+        context,
+        task_source,
+        canceller: Some(canceller),
     };
 
     ROUTER.add_route(
@@ -623,11 +626,10 @@ impl HTMLScriptElement {
 
     pub fn queue_error_event(&self) {
         let window = window_from_node(self);
-        window.dom_manipulation_task_source().queue_simple_event(
-            self.upcast(),
-            atom!("error"),
-            &window,
-        );
+        window
+            .task_manager()
+            .dom_manipulation_task_source()
+            .queue_simple_event(self.upcast(), atom!("error"), &window);
     }
 
     pub fn dispatch_load_event(&self) {