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/XHR/ProgressEvent.h

@@ -35,6 +35,8 @@ public:
 private:
     ProgressEvent(JS::Realm&, DeprecatedFlyString const& event_name, ProgressEventInit const& event_init);
 
+    virtual void initialize(JS::Realm&) override;
+
     bool m_length_computable { false };
     u64 m_loaded { 0 };
     u64 m_total { 0 };