Mark gtest and gmock as testonly in GN.

components/bookmarks/browser/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("browser") {
   sources = [
     "base_bookmark_model_observer.cc",
     "base_bookmark_model_observer.h",
@@ skipping 43 matching lines @@
   ]
 
   if (toolkit_views) {
     sources += [
       "bookmark_node_data_views.cc",
     ]
   }
 }
 
 source_set("unit_tests") {
+  testonly = true
   sources = [
     "bookmark_codec_unittest.cc",
     "bookmark_expanded_state_tracker_unittest.cc",
     "bookmark_index_unittest.cc",
     "bookmark_model_unittest.cc",
     "bookmark_utils_unittest.cc",
   ]
 
   deps = [
     ":browser",
     "//testing/gtest",
   ]
 }