Mark gtest and gmock as testonly in GN.

components/cloud_devices/common/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("common") {
   output_name = "cloud_devices_common"
   sources = [
     "cloud_device_description.cc",
     "cloud_device_description.h",
     "cloud_device_description_consts.cc",
     "cloud_device_description_consts.h",
     "cloud_devices_switches.cc",
     "cloud_devices_switches.h",
     "cloud_devices_urls.cc",
     "cloud_devices_urls.h",
     "description_items.h",
     "description_items_inl.h",
     "printer_description.cc",
     "printer_description.h",
   ]
   deps = [
     "//base",
     "//net",
   ]
 }
 
 source_set("unit_tests") {
+  testonly = true
   sources = [
     "cloud_devices_urls_unittest.cc",
     "printer_description_unittest.cc",
   ]
 
   deps = [
     ":common",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }