Mojo: Add warning of invalid serialization results (in debug build).

mojo/public/interfaces/bindings/tests/BUILD.gn

 # Copyright 2014 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("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("test_interfaces") {
   sources = [
     "math_calculator.mojom",
     "sample_factory.mojom",
     "sample_import.mojom",
     "sample_import2.mojom",
     "sample_interfaces.mojom",
     "sample_service.mojom",
+    "serialization_test_structs.mojom",
     "test_structs.mojom",
     "validation_test_interfaces.mojom",
   ]
 }