LibWeb: Move setting of Web object prototypes to initialize()

This needs to happen before prototype/constructor intitialization can be
made lazy. Otherwise, GC could run during the C++ constructor and try to
collect the object currently being created.

Userland/Libraries/LibWeb/Fetch/Headers.h

@@ -60,6 +60,7 @@ private:
 
     Headers(JS::Realm&, JS::NonnullGCPtr<Infrastructure::HeaderList>);
 
+    virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(JS::Cell::Visitor&) override;
 
     WebIDL::ExceptionOr<bool> validate(Infrastructure::Header const&) const;