Mark gtest and gmock as testonly in GN.

device/bluetooth/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("//tools/grit/grit_rule.gni")
 
 config("bluetooth_config") {
   if (is_win) {
     ldflags = [
       # Despite MSDN stating that Bthprops.dll contains the
@@ skipping 160 matching lines @@
     "device_bluetooth_strings_th.pak",
     "device_bluetooth_strings_tr.pak",
     "device_bluetooth_strings_uk.pak",
     "device_bluetooth_strings_vi.pak",
     "device_bluetooth_strings_zh-CN.pak",
     "device_bluetooth_strings_zh-TW.pak",
   ]
 }
 
 static_library("mocks") {
+  testonly = true
   sources = [
     "test/mock_bluetooth_adapter.cc",
     "test/mock_bluetooth_adapter.h",
     "test/mock_bluetooth_device.cc",
     "test/mock_bluetooth_device.h",
     "test/mock_bluetooth_discovery_session.cc",
     "test/mock_bluetooth_discovery_session.h",
     "test/mock_bluetooth_gatt_characteristic.cc",
     "test/mock_bluetooth_gatt_characteristic.h",
     "test/mock_bluetooth_gatt_connection.cc",
     "test/mock_bluetooth_gatt_connection.h",
     "test/mock_bluetooth_gatt_descriptor.cc",
     "test/mock_bluetooth_gatt_descriptor.h",
     "test/mock_bluetooth_gatt_notify_session.cc",
     "test/mock_bluetooth_gatt_notify_session.h",
     "test/mock_bluetooth_gatt_service.cc",
     "test/mock_bluetooth_gatt_service.h",
     "test/mock_bluetooth_socket.cc",
     "test/mock_bluetooth_socket.h",
   ]
 
   deps = [
     ":bluetooth",
     "//testing/gmock",
   ]
 }