Mark gtest and gmock as testonly in GN.

components/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("//build/config/features.gni")
 
 # Collection of all components. You wouldn't link to this, but this is rather
 # to reference the files so they can be compiled by the build system.
 group("all_components") {
+  testonly = true  # You shouldn't actually link to this.
   visibility = [ "//:*" ]  # Only for the root targets to bring in.
 
   deps = [
     "//components/auto_login_parser",
     "//components/autofill/content/browser",
     "//components/autofill/content/common",
     "//components/autofill/content/renderer",
     "//components/bookmarks/browser",
     "//components/bookmarks/common",
     "//components/bookmarks/test",
@@ skipping 197 matching lines @@
   ]
 
   # TODO(GYP) need this target.
   #'breakpad/app/crash_keys_win_unittest.cc',
 
   # Precache tests need these defines.
   #configs += [ "//components/precache/core:precache_config" ]
 }
 
 }