Mark gtest and gmock as testonly in GN.

components/bookmarks/test/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/ui.gni")
 
 source_set("test") {
+  testonly = true
   sources = [
     "bookmark_test_helpers.cc",
     "bookmark_test_helpers.h",
     "mock_bookmark_model_observer.cc",
     "mock_bookmark_model_observer.h",
     "test_bookmark_client.cc",
     "test_bookmark_client.h",
   ]
 
   deps = [
     "//base",
     "//components/bookmarks/browser",
     "//testing/gmock",
     "//ui/events/platform",
     "//url",
   ]
 
   if (use_x11) {
     deps += [
       "//ui/events/platform/x11",
     ]
   }
 }