Add the "testonly" flag to the GN test target.

mojo/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")
 
 group("mojo") {
+  # Meta-target, don't link into production code.
+  testonly = true
   deps = [
     ":tests",
     "//mojo/common",
     "//mojo/examples",
     "//mojo/public",
     "//mojo/services",
     "//mojo/shell:mojo_shell",
   ]
 
   if (is_linux) {
     deps += [
       "//mojo/python",
     ]
   }
 }
 
 group("tests") {
+  testonly = true
   deps = [
     "//mojo/application_manager:mojo_application_manager_unittests",
     "//mojo/apps/js/test:mojo_apps_js_unittests",
     "//mojo/common:mojo_common_unittests",
     "//mojo/public/cpp/application/tests:mojo_public_application_unittests",
     "//mojo/public/cpp/bindings/tests:mojo_public_bindings_unittests",
     "//mojo/public/cpp/environment/tests:mojo_public_environment_unittests",
     "//mojo/public/cpp/system/tests:mojo_public_system_unittests",
     "//mojo/public/cpp/utility/tests:mojo_public_utility_unittests",
     "//mojo/public/js/bindings/tests:mojo_js_unittests",
@@ skipping 19 matching lines @@
     sources = [
       "android/javatests/src/org/chromium/mojo/MojoTestCase.java",
       "android/javatests/src/org/chromium/mojo/bindings/ValidationTestUtil.java",
       "android/system/src/org/chromium/mojo/system/impl/CoreImpl.java",
       "services/native_viewport/android/src/org/chromium/mojo/PlatformViewportAndroid.java",
     ]
 
     jni_package = "mojo"
   }
 }