net: Remove CoreResourceThread from FetchThread state (#38422)

In single process mode, there is a race condition on the initialization
of the global fetch thread: once initialized the global fetch thread
will always use a given core resource thread, and this will be
determined by the component who first initializes it. For example, if
the canvas paint thread first does an async fetch, then this will set
the public core resource as used for all future fetches, including those
coming from a pipeline in private mode.

In multi-process mode, there is a race condition per window event-loop:
the first pipeline to use the fetch will set the core resource thread
for all others.

To ensure the fetch thread uses the correct core resource thread(private
vs public), we need to
pass the core resource thread to each fetch thread operation for which
is it needed.

Testing: It should not break existing fetch WPT tests. The race
condition is not something that can be tested reliably, but it seems to
be based on solid logic.
Fixes: follow-up from
https://github.com/servo/servo/pull/38421/files#r2248950924

---------

Signed-off-by: gterzian <2792687+gterzian@users.noreply.github.com>

components/constellation/constellation.rs

@@ -733,7 +733,7 @@ where
         // Start a fetch thread.
         // In single-process mode this will be the global fetch thread;
         // in multi-process mode this will be used only by the canvas paint thread.
-        let join_handle = start_fetch_thread(&self.public_resource_threads.core_thread);
+        let join_handle = start_fetch_thread();
 
         while !self.shutting_down || !self.pipelines.is_empty() {
             // Randomly close a pipeline if --random-pipeline-closure-probability is set