Mojom cpp bindings: Enable non-nullable check.

mojo/public/interfaces/bindings/tests/sample_factory.mojom

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import "sample_import.mojom"
 
 [JavaPackage="org.chromium.mojo.bindings.test.mojom.sample"]
 module sample {
 
 // This sample shows how handles to MessagePipes can be sent as both parameters
 // to methods as well as fields on structs.
 
 struct Request {
   int32 x;
-  handle<message_pipe> pipe;
-  handle<message_pipe>[] more_pipes;
+  handle<message_pipe>? pipe;
+  handle<message_pipe>[]? more_pipes;
 
   // Interfaces can be used as members.
-  imported.ImportedInterface obj;
+  imported.ImportedInterface? obj;
 };
 
 struct Response {
   int32 x;
-  handle<message_pipe> pipe;
+  handle<message_pipe>? pipe;
 };
 
 interface NamedObject {
   SetName(string name);
   GetName() => (string name);
 };
 
 [Client=FactoryClient]
 interface Factory {
-  DoStuff(Request request, handle<message_pipe> pipe);
+  DoStuff(Request request, handle<message_pipe>? pipe);
   DoStuff2(handle<data_pipe_consumer> pipe);
   CreateNamedObject(NamedObject& obj);
   RequestImportedInterface(
       imported.ImportedInterface& obj) => (imported.ImportedInterface& obj);
   TakeImportedInterface(
       imported.ImportedInterface obj) => (imported.ImportedInterface obj);
 };
 
 interface FactoryClient {
   DidStuff(Response response, string text);
   DidStuff2(string text);
 };
 
 }  // module sample