Fetch cancellation
This PR implements cancellation for fetch, and uses it for XHR. This means that fetch clients can now send a message to the fetch task asking for the network request to be aborted.
Previously, clients like XHR had abort functionality but would implement it by simply ignoring future messages from the network task; and would not actually cancel the network fetch.
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http://www.robohornet.org gives a score of 101.36 on master,
and 102.68 with this PR. The latter is slightly better,
but probably within noise level.
So it looks like this PR does not affect DOM performance.
This is expected since `Box::new` is defined as:
```rust
impl<T> Box<T> {
#[inline(always)]
pub fn new(x: T) -> Box<T> {
box x
}
}
```
With inlining, it should compile to the same as box syntax.
In a later PR, DomRoot<T> will become a type alias of Root<Dom<T>>,
where Root<T> will be able to handle all the things that need to be
rooted that have a stable traceable address that doesn't move for the
whole lifetime of the root. Stay tuned.
The spec removed it. Check the scheme instead, data is always same origin now,
except for workers.
This also updates the comments to make step numbers match the spec.
Closes#13362
