LibJS: Add Object::set_prototype()

This is just factoring out step "9. Set O.[[Prototype]] to V." of
10.1.2 [[SetPrototypeOf]] into its own method so that we don't have to
use internal_set_prototype_of() for setting an object prototype in all
cases.

Userland/Libraries/LibJS/Runtime/Object.cpp

@@ -546,11 +546,7 @@ ThrowCompletionOr<bool> Object::internal_set_prototype_of(Object* new_prototype)
     }
 
     // 9. Set O.[[Prototype]] to V.
-    auto& shape = this->shape();
+    set_prototype(new_prototype);
-    if (shape.is_unique())
-        shape.set_prototype_without_transition(new_prototype);
-    else
-        m_shape = shape.create_prototype_transition(new_prototype);
 
     // 10. Return true.
     return true;
@@ -983,6 +979,17 @@ void Object::storage_delete(PropertyName const& property_name)
     m_storage.remove(metadata->offset);
 }
 
+void Object::set_prototype(Object* new_prototype)
+{
+    if (prototype() == new_prototype)
+        return;
+    auto& shape = this->shape();
+    if (shape.is_unique())
+        shape.set_prototype_without_transition(new_prototype);
+    else
+        m_shape = shape.create_prototype_transition(new_prototype);
+}
+
 void Object::define_native_accessor(PropertyName const& property_name, Function<Value(VM&, GlobalObject&)> getter, Function<Value(VM&, GlobalObject&)> setter, PropertyAttributes attribute)
 {
     auto& vm = this->vm();

Userland/Libraries/LibJS/Runtime/Object.h

@@ -171,6 +171,8 @@ protected:
     explicit Object(GlobalObjectTag);
     Object(ConstructWithoutPrototypeTag, GlobalObject&);
 
+    void set_prototype(Object*);
+
     // [[Extensible]]
     bool m_is_extensible { true };