Mark gtest and gmock as testonly in GN.

base/BUILD.gn

 # Copyright (c) 2013 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")
 
 if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
@@ skipping 1017 matching lines @@
 
   deps = [
     ":base",
     ":prefs",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }
 
 source_set("message_loop_tests") {
+  testonly = true
   sources = [
     "message_loop/message_loop_test.cc",
     "message_loop/message_loop_test.h",
   ]
 
   deps = [
     ":base",
     "//testing/gtest",
   ]
 }
@@ skipping 380 matching lines @@
 
   unittest_apk("base_unittests_apk") {
     deps = [
       ":base_java",
       ":base_java_unittest_support",
       ":base_unittests",
     ]
     unittests_dep = ":base_unittests"
   }
 }