Sync: Avoid 3 passes over SyncDarta DB when loading the directory from the disk

sync/syncable/deferred_on_disk_directory_backing_store.h

 // Copyright 2014 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 SYNC_SYNCABLE_DEFERRED_ON_DISK_DIRECTORY_BACKING_STORE_H_
 #define SYNC_SYNCABLE_DEFERRED_ON_DISK_DIRECTORY_BACKING_STORE_H_
 
 #include "base/files/file_path.h"
 #include "sync/base/sync_export.h"
 #include "sync/syncable/directory_backing_store.h"
 
 namespace syncer {
 namespace syncable {
 
 // Store used for backing up user's data before first sync. It creates an
 // in-memory store first and switch to on-disk store if SaveChanges() is
 // called, which only happens when SyncBackupManager is shut down and a
 // syncing backend is to be created. Thus we guarantee that user data is not
 // persisted until user is actually going to sync.
 class SYNC_EXPORT_PRIVATE DeferredOnDiskDirectoryBackingStore
     : public DirectoryBackingStore {
  public:
   DeferredOnDiskDirectoryBackingStore(const std::string& dir_name,
                                       const base::FilePath& backing_filepath);
   ~DeferredOnDiskDirectoryBackingStore() override;
   DirOpenResult Load(Directory::MetahandlesMap* handles_map,
                      JournalIndex* delete_journals,
+                     MetahandleSet* metahandles_to_purge,
                      Directory::KernelLoadInfo* kernel_load_info) override;
   bool SaveChanges(const Directory::SaveChangesSnapshot& snapshot) override;
 
  private:
   base::FilePath backing_filepath_;
 
   // Whether current DB is on disk.
   bool db_is_on_disk_;
 
   DISALLOW_COPY_AND_ASSIGN(DeferredOnDiskDirectoryBackingStore);
 };
 
 }  // namespace syncable
 }  // namespace syncer
 
 #endif  // SYNC_SYNCABLE_DEFERRED_ON_DISK_DIRECTORY_BACKING_STORE_H_