Enable ui_struct_traits_unittests on try bots.

services/ui/public/interfaces/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")
 import("//testing/test.gni")
 
 mojom("interfaces") {
   sources = [
     "accelerator_registrar.mojom",
@@ skipping 34 matching lines @@
     "//ui/events/mojo:interfaces",
     "//ui/gfx/geometry/mojo",
     "//ui/gfx/mojo",
     "//ui/platform_window/mojo:interfaces",
   ]
 
   use_new_wrapper_types = false
 }
 
 test("ui_struct_traits_unittests") {
+  testonly = true
+
   sources = [
     "display/display_struct_traits_unittest.cc",
   ]
 
   deps = [
     "//base",
     "//base/test:test_support",
     "//mojo/edk/test:run_all_unittests",
     "//services/ui/public/interfaces/display:test_interfaces",
     "//testing/gtest",
     "//ui/display/types",
     "//ui/gfx:test_support",
   ]
 }