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

sync/syncable/mutable_entry.cc

 // Copyright 2012 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.
 
 #include "sync/syncable/mutable_entry.h"
 
 #include "base/memory/scoped_ptr.h"
 #include "sync/internal_api/public/base/unique_position.h"
 #include "sync/syncable/directory.h"
 #include "sync/syncable/scoped_kernel_lock.h"
@@ skipping 166 matching lines @@
   write_transaction()->TrackChangesTo(kernel_);
   if (value == kernel_->ref(IS_DEL)) {
     return;
   }
   if (value) {
     // If the server never knew about this item and it's deleted then we don't
     // need to keep it around.  Unsetting IS_UNSYNCED will:
     // - Ensure that the item is never committed to the server.
     // - Allow any items with the same UNIQUE_CLIENT_TAG created on other
     //   clients to override this entry.
-    // - Let us delete this entry permanently through
-    //   DirectoryBackingStore::DropDeletedEntries() when we next restart sync.
-    //   This will save memory and avoid crbug.com/125381.
+    // - Let us delete this entry permanently when we next restart sync - see
+    //   DirectoryBackingStore::SafeToPurgeOnLoading.
+    //   This will avoid crbug.com/125381.
     // Note: do not unset IsUnsynced if the syncer is in the middle of
     // attempting to commit this entity.
     if (!GetId().ServerKnows() && !GetSyncing()) {
       PutIsUnsynced(false);
     }
   }
 
   {
     ScopedKernelLock lock(dir());
     // Some indices don't include deleted items and must be updated
@@ skipping 92 matching lines @@
   DCHECK_NE(static_cast<MutableEntry*>(NULL), e);
   DCHECK(!e->IsRoot()) << "We shouldn't mark a permanent object for syncing.";
   if (!(e->PutIsUnsynced(true)))
     return false;
   e->PutSyncing(false);
   return true;
 }
 
 }  // namespace syncable
 }  // namespace syncer