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

components/sync/driver/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 COMPONENTS_SYNC_DRIVER_SYNC_CLIENT_H_
 #define COMPONENTS_SYNC_DRIVER_SYNC_CLIENT_H_
 
 #include "base/callback_forward.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "components/sync/base/extensions_activity.h"
 #include "components/sync/base/model_type.h"
 #include "components/sync/driver/sync_api_component_factory.h"
 #include "components/sync/engine/model_safe_worker.h"
 #include "components/sync/model/model_type_sync_bridge.h"
 
 class BookmarkUndoService;
 class PrefService;
 
 namespace autofill {
 class PersonalDataManager;
 }  // namespace autofill
 
+namespace base {
+class SequencedWorkerPool;
+}  // namespace base
+
 namespace bookmarks {
 class BookmarkModel;
 }  // namespace bookmarks
 
 namespace favicon {
 class FaviconService;
 }  // namespace favicon
 
 namespace history {
 class HistoryService;
@@ skipping 19 matching lines @@
 // Note: on some platforms, getters might return nullptr. Callers are expected
 // to handle these scenarios gracefully.
 class SyncClient {
  public:
   SyncClient();
   virtual ~SyncClient();
 
   // Initializes the sync client with the specified sync service.
   virtual void Initialize() = 0;
 
+  // Returns SequencedWorkerPool to be used by ProfileSyncService for blocking
+  // operations.
+  virtual base::SequencedWorkerPool* GetBlockingPool() = 0;
+
   // Returns the current SyncService instance.
   virtual SyncService* GetSyncService() = 0;
 
   // Returns the current profile's preference service.
   virtual PrefService* GetPrefService() = 0;
 
   // Returns the path to the folder used for storing the local sync database.
   // It is only used when sync is running against a local backend.
   virtual base::FilePath GetLocalSyncBackendFolder() = 0;
 
@@ skipping 39 matching lines @@
   // Returns the current SyncApiComponentFactory instance.
   virtual SyncApiComponentFactory* GetSyncApiComponentFactory() = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(SyncClient);
 };
 
 }  // namespace syncer
 
 #endif  // COMPONENTS_SYNC_DRIVER_SYNC_CLIENT_H_