LibWeb: Invent a method to stop an in-progress fetch without errors

The HTMLMediaElement will need to stop fetching processes when its load
algorithm is invoked while a fetch is ongoing. We don't have a way to
really stop the process, due to the way it runs on nested deferred task
invocations. So for now, this swaps the fetch callbacks (e.g. to process
a fetch response) with empty callbacks.

Userland/Libraries/LibWeb/Fetch/Infrastructure/FetchParams.cpp

@@ -17,6 +17,7 @@ FetchParams::FetchParams(JS::NonnullGCPtr<Request> request, JS::NonnullGCPtr<Fet
     , m_controller(controller)
     , m_timing_info(timing_info)
 {
+    m_controller->set_fetch_params({}, *this);
 }
 
 JS::NonnullGCPtr<FetchParams> FetchParams::create(JS::VM& vm, JS::NonnullGCPtr<Request> request, JS::NonnullGCPtr<FetchTimingInfo> timing_info)