Mark gtest and gmock as testonly in GN.

components/password_manager/core/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.
 
 if (is_android) {
   import("//build/config/android/config.gni")
 }
 
 config("password_manager_config") {
   # Sync (not supported in Android WebView).
@@ skipping 74 matching lines @@
   if (!is_android || !is_android_webview_build) {
     sources += [
       "password_syncable_service.cc",
       "password_syncable_service.h",
     ]
     deps += [ "//sync" ]
   }
 }
 
 static_library("test_support") {
+  testonly = true
   sources = [
     "mock_password_store.cc",
     "mock_password_store.h",
     "password_form_data.cc",
     "password_form_data.h",
     "stub_password_manager_client.cc",
     "stub_password_manager_client.h",
     "stub_password_manager_driver.cc",
     "stub_password_manager_driver.h",
     "test_password_store.cc",
     "test_password_store.h",
   ]
 
   deps = [
     "//base",
     "//components/autofill/core/common",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }