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, 7 insertions, 5 deletions

diff --git a/components/script/layout_image.rs b/components/script/layout_image.rs
index 68a3e788cf4..39670eefabb 100644
--- a/components/script/layout_image.rs
+++ b/components/script/layout_image.rs
@@ -10,7 +10,6 @@
 use crate::dom::bindings::reflector::DomObject;
 use crate::dom::node::{document_from_node, Node};
 use crate::network_listener::{NetworkListener, PreInvoke};
-use crate::task_source::TaskSourceName;
 use ipc_channel::ipc;
 use ipc_channel::router::ROUTER;
 use net_traits::image_cache::{ImageCache, PendingImageId};
@@ -57,13 +56,16 @@ pub fn fetch_image_for_layout(
     }));
 
     let document = document_from_node(node);
-    let window = document.window();
 
     let (action_sender, action_receiver) = ipc::channel().unwrap();
+    let (task_source, canceller) = document
+        .window()
+        .task_manager()
+        .networking_task_source_with_canceller();
     let listener = NetworkListener {
-        context: context,
-        task_source: window.networking_task_source(),
-        canceller: Some(window.task_canceller(TaskSourceName::Networking)),
+        context,
+        task_source,
+        canceller: Some(canceller),
     };
     ROUTER.add_route(
         action_receiver.to_opaque(),