Mark gtest and gmock as testonly in GN.

components/autofill/core/common/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.
 
 static_library("common") {
   sources = [
     # TODO(brettw) why are these "browser" files in the "common" target?
     "../browser/android/auxiliary_profile_loader_android.cc",
     "../browser/android/auxiliary_profile_loader_android.h",
     "../browser/android/auxiliary_profiles_android.cc",
@@ skipping 37 matching lines @@
     "//ui/gfx",
     "//url",
   ]
 
   if (is_android) {
     # deps += [ 'autofill_jni_headers' ]  TODO(GYP)
   }
 }
 
 source_set("unit_tests") {
+  testonly = true
   sources = [
     "form_data_unittest.cc",
     "form_field_data_unittest.cc",
     "password_form_fill_data_unittest.cc",
     "save_password_progress_logger_unittest.cc",
   ]
 
   deps = [
     ":common",
     "//testing/gmock",
     "//testing/gtest",
   ]
 }