Mojo C++ bindings: support mapping T and T? differently.

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

 // Copyright 2016 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.
 
 module mojo.test;
 
 // TODO(yzshen): Rename *WithTraits* types to something more readable.
 
 struct NestedStructWithTraits {
   int32 value;
@@ skipping 31 matching lines @@
 struct MoveOnlyStructWithTraits {
   handle f_handle;
 };
 
 // The custom type for MoveOnlyStructWithTraits is not clonable. Test that
 // this container can compile as long as Clone() is not used.
 struct MoveOnlyStructWithTraitsContainer {
   MoveOnlyStructWithTraits f_struct;
 };
 
-struct StructWithTraitsForUniquePtrTest {
+struct StructWithTraitsForUniquePtr {
   int32 f_int32;
 };
 
 interface TraitsTestService {
   EchoStructWithTraits(StructWithTraits s) => (StructWithTraits passed);
 
   EchoTrivialStructWithTraits(TrivialStructWithTraits s) =>
       (TrivialStructWithTraits passed);
 
   EchoMoveOnlyStructWithTraits(MoveOnlyStructWithTraits s) =>
       (MoveOnlyStructWithTraits passed);
 
+  EchoNullableMoveOnlyStructWithTraits(MoveOnlyStructWithTraits? s) =>
+      (MoveOnlyStructWithTraits? passed);
+
   EchoEnumWithTraits(EnumWithTraits e) => (EnumWithTraits passed);
 
-  EchoStructWithTraitsForUniquePtrTest(StructWithTraitsForUniquePtrTest e) => (
-      StructWithTraitsForUniquePtrTest passed);
+  EchoStructWithTraitsForUniquePtr(StructWithTraitsForUniquePtr e) => (
+      StructWithTraitsForUniquePtr passed);
+
+  EchoNullableStructWithTraitsForUniquePtr(StructWithTraitsForUniquePtr? e) => (
+      StructWithTraitsForUniquePtr? passed);
 };