net: Use RequestId to cancel fetches instead of creating an IPC channel (#34883)

Instead of creating an IPC channel for every fetch, allow cancelling
fetches based on the `RequestId` of the original request. This requires
that `RequestId`s be UUIDs so that they are unique between processes
that might communicating with the resource process.

In addition, the resource process loop now keeps a `HashMap` or `Weak`
handles to cancellers and cleans them up.

This allows for creating mutiple `FetchCanceller`s in `script` for a
single fetch request, allowing integration of the media and video
elements to integrate with the `Document` canceller list -- meaning
these fetches also get cancelled when the `Document` unloads.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

components/script/dom/htmllinkelement.rs

@@ -384,7 +384,7 @@ impl HTMLLinkElement {
             resource_timing: ResourceFetchTiming::new(ResourceTimingType::Resource),
         };
 
-        document.fetch_background(request, fetch_context, None);
+        document.fetch_background(request, fetch_context);
     }
 
     /// <https://html.spec.whatwg.org/multipage/#concept-link-obtain>