LibJS: Make the GC marking phase cycle-proof

Don't visit cells that are already marked. This prevents the marking
phase from looping forever when two cells refer to each other.

Also do the marking directly from the CellVisitor, removing another
unnecessary phase of the collector. :^)

Libraries/LibJS/Cell.h

@@ -44,13 +44,10 @@ public:
 
     class Visitor {
     public:
-        virtual void did_visit(Cell*) = 0;
+        virtual void visit(Cell*) = 0;
     };
 
-    virtual void visit_graph(Visitor& visitor)
-    {
-        visitor.did_visit(this);
-    }
+    virtual void visit_children(Visitor&) {}
 
 private:
     bool m_mark { false };