[Hotword] Uninstall and reinstall the extension upon language change.

chrome/browser/search/hotword_service_factory.cc

 // Copyright 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.
 
 #include "chrome/browser/search/hotword_service_factory.h"
 
 #include "base/prefs/pref_service.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/search/hotword_service.h"
 #include "chrome/common/pref_names.h"
@@ skipping 85 matching lines @@
     user_prefs::PrefRegistrySyncable* prefs) {
   prefs->RegisterBooleanPref(prefs::kHotwordSearchEnabled,
                              false,
                              user_prefs::PrefRegistrySyncable::SYNCABLE_PREF);
   // Although this is default true, users will not send back information to
   // Google unless they have opted in to Hotwording at which point they must
   // also confirm that they wish this preference to be true or opt out of it.
   prefs->RegisterBooleanPref(prefs::kHotwordAudioLoggingEnabled,
                              true,
                              user_prefs::PrefRegistrySyncable::SYNCABLE_PREF);
+  prefs->RegisterStringPref(prefs::kHotwordPreviousLanguage,
+                            std::string(),
+                            user_prefs::PrefRegistrySyncable::SYNCABLE_PREF);
 }
 
 KeyedService* HotwordServiceFactory::BuildServiceInstanceFor(
     BrowserContext* context) const {
   return new HotwordService(Profile::FromBrowserContext(context));
 }