Mark gtest and gmock as testonly in GN.

components/crx_file/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.
 
 source_set("crx_file") {
   sources = [
     "constants.h",
     "crx_file.cc",
     "crx_file.h",
     "id_util.cc",
     "id_util.h",
   ]
 
   deps = [
     "//base",
   ]
 }
 
 source_set("unit_tests") {
+  testonly = true
   sources = [
     "id_util_unittest.cc",
   ]
 
   deps = [
     ":crx_file",
     "//testing/gtest",
   ]
 }