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/HTML/CanvasRenderingContext2D.h

@@ -86,6 +86,7 @@ public:
 private:
     explicit CanvasRenderingContext2D(JS::Realm&, HTMLCanvasElement&);
 
+    virtual void initialize(JS::Realm&) override;
     virtual void visit_edges(Cell::Visitor&) override;
 
     struct PreparedTextGlyph {