Mark gtest and gmock as testonly in GN.

components/captive_portal/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.
 
 component("captive_portal") {
   sources = [
     "captive_portal_detector.cc",
     "captive_portal_detector.h",
     "captive_portal_types.cc",
     "captive_portal_types.h",
     "captive_portal_export.h",
   ]
 
   defines = [ "CAPTIVE_PORTAL_IMPLEMENTATION" ]
 
   deps = [
     "//base",
     "//net",
     "//url",
   ]
 }
 
 static_library("test_support") {
+  testonly = true
   sources = [
     "captive_portal_testing_utils.cc",
     "captive_portal_testing_utils.h",
   ]
 
   deps = [
     ":captive_portal",
     "//base",
     "//net",
     "//url",
   ]
 }
 
 source_set("unit_tests") {
+  testonly = true
   sources = [
     "captive_portal_detector_unittest.cc",
   ]
 
   deps = [
     ":captive_portal",
     "//testing/gtest",
   ]
 }