Update the app banner action button to display "add" by default.

chrome/browser/banners/app_banner_settings_helper.cc

 // 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.
 
 #include "chrome/browser/banners/app_banner_settings_helper.h"
 
 #include <stddef.h>
 
 #include <memory>
 #include <string>
@@ skipping 383 matching lines @@
 
 AppBannerSettingsHelper::LanguageOption
 AppBannerSettingsHelper::GetHomescreenLanguageOption() {
   std::string param = variations::GetVariationParamValue(
       kBannerParamsKey, kBannerParamsLanguageKey);
   unsigned int language_option = 0;
 
   if (param.empty() || !base::StringToUint(param, &language_option) ||
       language_option < LANGUAGE_OPTION_MIN ||
       language_option > LANGUAGE_OPTION_MAX) {
-    return LANGUAGE_OPTION_DEFAULT;
+    return LANGUAGE_OPTION_ADD;
   }
 
   return static_cast<LanguageOption>(language_option);
 }