Add a switch in BUILD rules to control the generation of type info for mojom files.

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("../../../tools/bindings/mojom.gni")
 
 mojom("test_interfaces") {
   testonly = true
+  generate_type_info = true
   sources = [
     "math_calculator.mojom",
     "no_module.mojom",
     "ping_service.mojom",
     "rect.mojom",
     "regression_tests.mojom",
     "sample_factory.mojom",
     "sample_import.mojom",
     "sample_import2.mojom",
     "sample_interfaces.mojom",
@@ skipping 17 matching lines @@
   ]
 }
 
 mojom("versioning_test_client_interfaces") {
   # FIXME: Dart packaged applications cannot depend on testonly mojoms.
   # testonly = true
   sources = [
     "versioning_test_client.mojom",
   ]
 }