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, 7 deletions

diff --git a/components/script/script_thread.rs b/components/script/script_thread.rs
index c4177500b64..fe102a0a7b3 100644
--- a/components/script/script_thread.rs
+++ b/components/script/script_thread.rs
@@ -70,6 +70,7 @@ use crate::microtask::{Microtask, MicrotaskQueue};
 use crate::script_runtime::{get_reports, new_rt_and_cx, Runtime, ScriptPort};
 use crate::script_runtime::{CommonScriptMsg, ScriptChan, ScriptThreadEventCategory};
 use crate::serviceworkerjob::{Job, JobQueue};
+use crate::task_manager::TaskManager;
 use crate::task_queue::{QueuedTask, QueuedTaskConversion, TaskQueue};
 use crate::task_source::dom_manipulation::DOMManipulationTaskSource;
 use crate::task_source::file_reading::FileReadingTaskSource;
@@ -2583,20 +2584,23 @@ impl ScriptThread {
             pipeline_id: incomplete.pipeline_id,
         };
 
-        // Create the window and document objects.
-        let window = Window::new(
-            self.js_runtime.clone(),
-            MainThreadScriptChan(sender.clone()),
+        let task_manager = TaskManager::new(
             self.dom_manipulation_task_source(incomplete.pipeline_id),
+            self.file_reading_task_source(incomplete.pipeline_id),
+            self.history_traversal_task_source(incomplete.pipeline_id),
             self.media_element_task_source(incomplete.pipeline_id),
-            self.user_interaction_task_source(incomplete.pipeline_id),
             self.networking_task_source(incomplete.pipeline_id),
-            self.history_traversal_task_source(incomplete.pipeline_id),
-            self.file_reading_task_source(incomplete.pipeline_id),
             self.performance_timeline_task_source(incomplete.pipeline_id)
                 .clone(),
+            self.user_interaction_task_source(incomplete.pipeline_id),
             self.remote_event_task_source(incomplete.pipeline_id),
             self.websocket_task_source(incomplete.pipeline_id),
+        );
+        // Create the window and document objects.
+        let window = Window::new(
+            self.js_runtime.clone(),
+            MainThreadScriptChan(sender.clone()),
+            task_manager,
             self.image_cache_channel.clone(),
             self.image_cache.clone(),
             self.resource_threads.clone(),