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

ios/chrome/browser/sync/ios_chrome_sync_client.mm

 // 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 "ios/chrome/browser/sync/ios_chrome_sync_client.h"
 
 #include <utility>
 
 #include "base/bind.h"
 #include "base/command_line.h"
@@ skipping 158 matching lines @@
         ui::GetDeviceFormFactor() == ui::DEVICE_FORM_FACTOR_TABLET,
         *base::CommandLine::ForCurrentProcess(),
         prefs::kSavingBrowserHistoryDisabled, sync_service_url,
         web::WebThread::GetTaskRunnerForThread(web::WebThread::UI),
         web::WebThread::GetTaskRunnerForThread(web::WebThread::DB),
         token_service, url_request_context_getter, web_data_service_,
         password_store_));
   }
 }
 
+base::SequencedWorkerPool* IOSChromeSyncClient::GetBlockingPool() {
+  return web::WebThread::GetBlockingPool();
+}
+
 syncer::SyncService* IOSChromeSyncClient::GetSyncService() {
   DCHECK_CURRENTLY_ON(web::WebThread::UI);
   return IOSChromeProfileSyncServiceFactory::GetForBrowserState(browser_state_);
 }
 
 PrefService* IOSChromeSyncClient::GetPrefService() {
   DCHECK_CURRENTLY_ON(web::WebThread::UI);
   return browser_state_->GetPrefs();
 }
 
@@ skipping 232 matching lines @@
         IOSChromeProfileSyncServiceFactory::GetForBrowserState(browser_state);
     if (profile_sync_service != nullptr) {
       const syncer::DeviceInfoTracker* tracker =
           profile_sync_service->GetDeviceInfoTracker();
       if (tracker != nullptr) {
         trackers->push_back(tracker);
       }
     }
   }
 }