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

sync/syncable/directory.h

 // Copyright 2013 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_DIRECTORY_H_
 #define SYNC_SYNCABLE_DIRECTORY_H_
 
 #include <deque>
 #include <set>
 #include <string>
@@ skipping 581 matching lines @@
 
   // Purges from memory any unused, safe to remove entries that were
   // successfully deleted on disk as a result of the SaveChanges that processed
   // |snapshot|.  See SaveChanges() for more information.
   bool VacuumAfterSaveChanges(const SaveChangesSnapshot& snapshot);
 
   // Rolls back dirty bits in the event that the SaveChanges that
   // processed |snapshot| failed, for example, due to no disk space.
   void HandleSaveChangesFailure(const SaveChangesSnapshot& snapshot);
 
-  // Used by CheckTreeInvariants
+  // Used by CheckTreeInvariants.
   void GetAllMetaHandles(BaseTransaction* trans, MetahandleSet* result);
+
+  // Used by VacuumAfterSaveChanges.
   bool SafeToPurgeFromMemory(WriteTransaction* trans,
                              const EntryKernel* const entry) const;
-
   // A helper used by GetTotalNodeCount.
   void GetChildSetForKernel(
       BaseTransaction*,
       EntryKernel* kernel_,
       std::deque<const OrderedChildSet*>* child_sets) const;
 
   // Append the handles of the children of |parent_id| to |result|.
   void AppendChildHandles(const ScopedKernelLock& lock,
                           const Id& parent_id,
                           Directory::Metahandles* result);
@@ skipping 30 matching lines @@
   // are deleted in native models as well.
   scoped_ptr<DeleteJournal> delete_journal_;
 
   DISALLOW_COPY_AND_ASSIGN(Directory);
 };
 
 }  // namespace syncable
 }  // namespace syncer
 
 #endif // SYNC_SYNCABLE_DIRECTORY_H_