Mark gtest and gmock as testonly in GN.

components/domain_reliability/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.
 
 action("bake_in_configs") {
   visibility = [ ":*" ]
   script = "bake_in_configs.py"
 
   inputs = [
     "baked_in_configs/apis_google_com.json",
@@ skipping 44 matching lines @@
     ":bake_in_configs",
     "//base",
     "//components/keyed_service/core",
     "//content/public/browser",
     "//net",
     "//url",
   ]
 }
 
 source_set("unit_tests") {
+  testonly = true
   sources = [
     "config_unittest.cc",
     "context_unittest.cc",
     "dispatcher_unittest.cc",
     "monitor_unittest.cc",
     "scheduler_unittest.cc",
     "test_util.cc",
     "test_util.h",
     "uploader_unittest.cc",
     "util_unittest.cc",
   ]
 
   deps = [
     ":domain_reliability",
     "//testing/gtest",
   ]
 }