LibWeb: Register PendingResponse with a Request to keep it alive

This was an oversight from when I converted PendingResponse and various
other classes from being ref-counted to GC-allocated last minute - no
one takes care to keep all of them alive. Some are on the stack, and
some might be captured in another PendingResponse's JS::SafeFunction,
but ultimately, we need a better solution.
Since a PendingResponse is *always* the result of someone having created
a Request, let's just let that keep a list of each PendingResponse that
has been created for it, and visit them until they are resolved. After
that, they can be GC'd with no complaints.

Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Requests.cpp

@@ -7,6 +7,7 @@
 #include <AK/Array.h>
 #include <LibJS/Heap/Heap.h>
 #include <LibJS/Runtime/Realm.h>
+#include <LibWeb/Fetch/Fetching/PendingResponse.h>
 #include <LibWeb/Fetch/Infrastructure/HTTP/Requests.h>
 #include <LibWeb/URL/URL.h>
 
@@ -21,6 +22,8 @@ void Request::visit_edges(JS::Cell::Visitor& visitor)
 {
     Base::visit_edges(visitor);
     visitor.visit(m_header_list);
+    for (auto pending_response : m_pending_responses)
+        visitor.visit(pending_response);
 }
 
 JS::NonnullGCPtr<Request> Request::create(JS::VM& vm)