Allow immutable prototype templates to inherit

To enable the global object prototype chain to be frozen, all objects
in the chain need to be marked as immutable prototype exotic objects.
However, a bug in the previous implementation of immutable prototype
exotic objects left the check in place when initially setting up the
object, which made it impossible to allow inheritance chains. This
patch removes that mistaken check.

BUG=v8:5149

Review-Url: https://codereview.chromium.org/2449163004
Cr-Commit-Position: refs/heads/master@{#40702}

src/api-natives.cc

@@ -437,9 +437,7 @@ MaybeHandle<JSFunction> InstantiateFunction(Isolate* isolate,
           JSObject::GetProperty(parent_instance,
                                 isolate->factory()->prototype_string()),
           JSFunction);
-      MAYBE_RETURN(JSObject::SetPrototype(prototype, parent_prototype, false,
-                                          Object::THROW_ON_ERROR),
-                   MaybeHandle<JSFunction>());
+      JSObject::ForceSetPrototype(prototype, parent_prototype);
     }
   }
   Handle<JSFunction> function = ApiNatives::CreateApiFunction(

test/cctest/test-api.cc

@@ -25939,3 +25939,41 @@ TEST(EvalInAccessCheckedContext) {
   CHECK_EQ(42, x_value->Int32Value(context1).FromJust());
   context1->Exit();
 }
+
+THREADED_TEST(ImmutableProtoWithParent) {
+  LocalContext context;
+  v8::Isolate* isolate = context->GetIsolate();
+  v8::HandleScope handle_scope(isolate);
+
+  Local<v8::FunctionTemplate> parent = v8::FunctionTemplate::New(isolate);
+
+  Local<v8::FunctionTemplate> templ = v8::FunctionTemplate::New(isolate);
+  templ->Inherit(parent);
+  templ->PrototypeTemplate()->SetImmutableProto();
+
+  Local<v8::Function> function =
+      templ->GetFunction(context.local()).ToLocalChecked();
+  Local<v8::Object> instance =
+      function->NewInstance(context.local()).ToLocalChecked();
+  Local<v8::Object> prototype =
+      instance->Get(context.local(), v8_str("__proto__"))
+          .ToLocalChecked()
+          ->ToObject(context.local())
+          .ToLocalChecked();
+
+  // Look up the prototype
+  Local<v8::Value> original_proto =
+      prototype->Get(context.local(), v8_str("__proto__")).ToLocalChecked();
+
+  // Setting the prototype (e.g., to null) throws
+  CHECK(
+      prototype->SetPrototype(context.local(), v8::Null(isolate)).IsNothing());
+
+  // The original prototype is still there
+  Local<Value> new_proto =
+      prototype->Get(context.local(), v8_str("__proto__")).ToLocalChecked();
+  CHECK(new_proto->IsObject());
+  CHECK(new_proto.As<v8::Object>()
+            ->Equals(context.local(), original_proto)
+            .FromJust());
+}