Mark gtest and gmock as testonly in GN.

components/sync_driver/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.
 
 static_library("sync_driver") {
   sources = [
     "backend_data_type_configurer.cc",
     "backend_data_type_configurer.h",
     "change_processor.cc",
     "change_processor.h",
@@ skipping 43 matching lines @@
   ]
 
   deps = [
     "//base",
     "//components/os_crypt",
     "//sync",
   ]
 }
 
 static_library("test_support") {
+  testonly = true
   sources = [
     "change_processor_mock.cc",
     "change_processor_mock.h",
     "data_type_controller_mock.cc",
     "data_type_controller_mock.h",
     "data_type_error_handler_mock.cc",
     "data_type_error_handler_mock.h",
     "data_type_manager_mock.cc",
     "data_type_manager_mock.h",
     "fake_data_type_controller.cc",
     "fake_data_type_controller.h",
     "fake_generic_change_processor.cc",
     "fake_generic_change_processor.h",
     "model_associator_mock.cc",
     "model_associator_mock.h",
     "non_ui_data_type_controller_mock.cc",
     "non_ui_data_type_controller_mock.h",
   ]
 
   deps = [
     ":sync_driver",
     "//base",
     "//sync",
     "//sync:test_support_sync_internal_api",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }