Add lots of testonly annotations to the GN build.

mojo/services/public/cpp/view_manager/lib/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("//build/config/ui.gni")
 
 # GYP version: mojo/mojo_services.gypi:mojo_view_manager_run_unittests
 source_set("run_unittests") {
+  testonly = true
   sources = [
     "view_manager_test_suite.cc",
     "view_manager_test_suite.h",
     "view_manager_unittests.cc",
   ]
 
   deps = [
     "//base",
     "//base/test:test_support",
     "//ui/gl",
   ]
   if (is_linux || is_win) {
     deps += [
       "//third_party/mesa:osmesa",
       "//mojo/services/native_viewport",
     ]
   }
   if (use_x11) {
     deps += [ "//ui/gfx/x" ]
   }
 }