[Sync] ModelTypeStore factory shouldn't require valid PSS to function correctly

chrome/browser/sync/chrome_sync_client.cc

 // Copyright 2015 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/sync/chrome_sync_client.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/command_line.h"
@@ skipping 218 matching lines @@
         prefs::kSavingBrowserHistoryDisabled, sync_service_url,
         content::BrowserThread::GetTaskRunnerForThread(
             content::BrowserThread::UI),
         content::BrowserThread::GetTaskRunnerForThread(
             content::BrowserThread::DB),
         token_service, url_request_context_getter, web_data_service_,
         password_store_);
   }
 }
 
+base::SequencedWorkerPool* ChromeSyncClient::GetBlockingPool() {
+  return content::BrowserThread::GetBlockingPool();
+}
+
 syncer::SyncService* ChromeSyncClient::GetSyncService() {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   return ProfileSyncServiceFactory::GetSyncServiceForBrowserContext(profile_);
 }
 
 PrefService* ChromeSyncClient::GetPrefService() {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
   return profile_->GetPrefs();
 }
 
@@ skipping 443 matching lines @@
   sync_service->RegisterDataTypeController(
       base::MakeUnique<SupervisedUserSyncDataTypeController>(
           syncer::SUPERVISED_USER_SETTINGS, error_callback, this, profile_));
   sync_service->RegisterDataTypeController(
       base::MakeUnique<SupervisedUserSyncDataTypeController>(
           syncer::SUPERVISED_USER_WHITELISTS, error_callback, this, profile_));
 #endif
 }
 
 }  // namespace browser_sync