script: Have FetchResponseListener::process_response_eof consume the listener (#40556)

The goal of this change is to prevent having to copy so much data out of
listeners when a fetch completes, which will be particularly important
for off-the-main thread parsing of CSS (see #22478). This change has
pros and cons:

Pros:
- This makes the design of the `FetchResponseListener` a great deal
simpler.
They no longer individually store a dummy `ResourceFetchTiming` that is
   only replaced right before `process_response_eof`.
 - The creation of the `Arc<Mutex<FetchResponseListener>>` in the
   `NetworkListener` is abstracted away from clients and now they just
   pass the `FetchResponseListener` to the fetch methods in the global.

Cons:
 - Now each `FetchResponseListener` must explicitly call `submit_timing`
   instead of having the `NetworkListener` do it. This is arguably a bit
   easier to follow in the code.
 - Since the internal data of the `NetworkListener` is now an
   `Arc<Mutex<Option<FetchResponseListener>>>`, when the fetching code
   needs to share state with the `NetworkListener` it either needs to
   share an `Option` or some sort of internal state. In one case I've
   stored the `Option` and in another case, I've stored a new inner
   shared value.

Testing: This should not change observable behavior and is thus covered
by existing tests.
Fixes: #22550

---------

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

components/script/script_thread.rs

@@ -3657,8 +3657,8 @@ impl ScriptThread {
             .position(|&(pipeline_id, _)| pipeline_id == id);
 
         if let Some(idx) = idx {
-            let (_, mut ctxt) = self.incomplete_parser_contexts.0.borrow_mut().remove(idx);
-            ctxt.process_response_eof(request_id, eof);
+            let (_, context) = self.incomplete_parser_contexts.0.borrow_mut().remove(idx);
+            context.process_response_eof(request_id, eof);
         }
     }