LibJS: Add the WeakRef built-in object

Userland/Libraries/LibJS/Runtime/WeakRef.h

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2021, Idan Horowitz <idan.horowitz@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Runtime/GlobalObject.h>
+#include <LibJS/Runtime/Object.h>
+#include <LibJS/Runtime/WeakContainer.h>
+
+namespace JS {
+
+class WeakRef final
+    : public Object
+    , public WeakContainer {
+    JS_OBJECT(WeakRef, Object);
+
+public:
+    static WeakRef* create(GlobalObject&, Object*);
+
+    explicit WeakRef(Object& prototype, Object*);
+    virtual ~WeakRef() override;
+
+    Object* value() const { return m_value; };
+
+    void update_execution_generation() { m_last_execution_generation = vm().execution_generation(); };
+
+    virtual void remove_sweeped_cells(Badge<Heap>, Vector<Cell*>&) override;
+
+private:
+    virtual void visit_edges(Visitor&) override;
+
+    Object* m_value { nullptr };
+    u32 m_last_execution_generation { 0 };
+};
+
+}