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/FetchController.h

@@ -6,11 +6,13 @@
 
 #pragma once
 
+#include <AK/Badge.h>
 #include <LibJS/Forward.h>
 #include <LibJS/Heap/Cell.h>
 #include <LibJS/Heap/GCPtr.h>
 #include <LibJS/SafeFunction.h>
 #include <LibWeb/Fetch/Infrastructure/FetchTimingInfo.h>
+#include <LibWeb/Forward.h>
 
 namespace Web::Fetch::Infrastructure {
 
@@ -39,6 +41,10 @@ public:
     void abort(JS::Realm&, Optional<JS::Value>);
     void terminate();
 
+    void set_fetch_params(Badge<FetchParams>, JS::NonnullGCPtr<FetchParams> fetch_params) { m_fetch_params = fetch_params; }
+
+    void stop_fetch();
+
 private:
     FetchController();
 
@@ -67,6 +73,8 @@ private:
     // next manual redirect steps (default null)
     //     Null or an algorithm accepting nothing.
     Optional<JS::SafeFunction<void()>> m_next_manual_redirect_steps;
+
+    JS::GCPtr<FetchParams> m_fetch_params;
 };
 
 }