Mark gtest and gmock as testonly in GN.

gin/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("gin") {
   sources = [
     "arguments.cc",
     "arguments.h",
     "array_buffer.cc",
     "array_buffer.h",
@@ skipping 63 matching lines @@
 
   deps = [
     ":gin",
     "//base",
     "//base:i18n",
     "//v8",
   ]
 }
 
 source_set("gin_test") {
+  testonly = true
   sources = [
     "test/file.cc",
     "test/file.h",
     "test/file_runner.cc",
     "test/file_runner.h",
     "test/gc.cc",
     "test/gc.h",
     "test/gtest.cc",
     "test/gtest.h",
     "test/v8_test.cc",
@@ skipping 25 matching lines @@
     "test/run_js_tests.cc",
     "wrappable_unittest.cc",
   ]
 
   deps = [
     ":gin_test",
     "//base/test:test_support",
     "//v8",
   ]
 }