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

chrome/browser/sync/chrome_sync_client.h

 // 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.
 
 #ifndef CHROME_BROWSER_SYNC_CHROME_SYNC_CLIENT_H__
 #define CHROME_BROWSER_SYNC_CHROME_SYNC_CLIENT_H__
 
 #include <memory>
 #include <vector>
 
@@ skipping 19 matching lines @@
 
 namespace browser_sync {
 
 class ChromeSyncClient : public syncer::SyncClient {
  public:
   explicit ChromeSyncClient(Profile* profile);
   ~ChromeSyncClient() override;
 
   // SyncClient implementation.
   void Initialize() override;
+  base::SequencedWorkerPool* GetBlockingPool() override;
   syncer::SyncService* GetSyncService() override;
   PrefService* GetPrefService() override;
   base::FilePath GetLocalSyncBackendFolder() override;
   bookmarks::BookmarkModel* GetBookmarkModel() override;
   favicon::FaviconService* GetFaviconService() override;
   history::HistoryService* GetHistoryService() override;
   bool HasPasswordStore() override;
   base::Closure GetPasswordStateChangedCallback() override;
   syncer::SyncApiComponentFactory::RegisterDataTypesMethod
   GetRegisterPlatformTypesCallback() override;
@@ skipping 51 matching lines @@
   ExtensionsActivityMonitor extensions_activity_monitor_;
 
   base::WeakPtrFactory<ChromeSyncClient> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ChromeSyncClient);
 };
 
 }  // namespace browser_sync
 
 #endif  // CHROME_BROWSER_SYNC_CHROME_SYNC_CLIENT_H__