Mark gtest and gmock as testonly in GN.

ipc/BUILD.gn

 # Copyright (c) 2012 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.
 
 component("ipc") {
   sources = [
     "file_descriptor_set_posix.cc",
     "file_descriptor_set_posix.h",
     "ipc_channel.cc",
     "ipc_channel.h",
@@ skipping 140 matching lines @@
       "//base",
       "//base:i18n",
       "//base/test:test_support",
       "//base/test:test_support_perf",
       "//testing/gtest",
     ]
   }
 }
 
 static_library("test_support") {
+  testonly = true
   sources = [
     "ipc_multiprocess_test.cc",
     "ipc_multiprocess_test.h",
     "ipc_test_sink.cc",
     "ipc_test_sink.h",
     "ipc_test_base.cc",
     "ipc_test_base.h",
     "ipc_test_channel_listener.h",
     "ipc_test_channel_listener.cc",
   ]
   deps = [
     ":ipc",
     "//base",
     "//testing/gtest",
   ]
 }