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

sync/syncable/directory.h

Index: sync/syncable/directory.h
diff --git a/sync/syncable/directory.h b/sync/syncable/directory.h
index 98706b61cece207234d38f593d30ca6921a01394..89eec170c5050ae0d605851e8d07a268f20a26dc 100644
--- a/sync/syncable/directory.h
+++ b/sync/syncable/directory.h
@@ -599,11 +599,12 @@ class SYNC_EXPORT Directory {
   // 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*,