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, 17 deletions

diff --git a/components/script/task_source/history_traversal.rs b/components/script/task_source/history_traversal.rs
index ee73e7e991e..aa6877c9379 100644
--- a/components/script/task_source/history_traversal.rs
+++ b/components/script/task_source/history_traversal.rs
@@ -2,28 +2,16 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+use crate::script_runtime::{CommonScriptMsg, ScriptThreadEventCategory};
+use crate::script_thread::MainThreadScriptMsg;
+use crate::task::{TaskCanceller, TaskOnce};
+use crate::task_source::{TaskSource, TaskSourceName};
 use msg::constellation_msg::PipelineId;
-use script_runtime::{CommonScriptMsg, ScriptChan, ScriptThreadEventCategory};
-use script_thread::MainThreadScriptMsg;
 use servo_channel::Sender;
-use task::{TaskCanceller, TaskOnce};
-use task_source::{TaskSource, TaskSourceName};
 
-#[derive(JSTraceable)]
+#[derive(Clone, JSTraceable)]
 pub struct HistoryTraversalTaskSource(pub Sender<MainThreadScriptMsg>, pub PipelineId);
 
-impl ScriptChan for HistoryTraversalTaskSource {
-    fn send(&self, msg: CommonScriptMsg) -> Result<(), ()> {
-        self.0
-            .send(MainThreadScriptMsg::Common(msg))
-            .map_err(|_| ())
-    }
-
-    fn clone(&self) -> Box<ScriptChan + Send> {
-        Box::new(HistoryTraversalTaskSource((&self.0).clone(), (&self.1).clone()))
-    }
-}
-
 impl TaskSource for HistoryTraversalTaskSource {
     const NAME: TaskSourceName = TaskSourceName::HistoryTraversal;