[SyncFS] Fix backgrounding race of ConflictResolver

chrome/browser/sync_file_system/drive_backend/conflict_resolver.cc

 // 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.
 
 #include "chrome/browser/sync_file_system/drive_backend/conflict_resolver.h"
 
 #include "base/callback.h"
 #include "base/format_macros.h"
 #include "base/location.h"
 #include "base/logging.h"
@@ skipping 31 matching lines @@
 }
 
 void ConflictResolver::RunExclusive(scoped_ptr<SyncTaskToken> token) {
   if (!IsContextReady()) {
     SyncTaskManager::NotifyTaskDone(token.Pass(), SYNC_STATUS_FAILED);
     return;
   }
 
   // Conflict resolution should be invoked on clean tree.
   if (metadata_database()->HasDirtyTracker()) {
-    NOTREACHED();
-    SyncTaskManager::NotifyTaskDone(token.Pass(), SYNC_STATUS_FAILED);
+    SyncTaskManager::NotifyTaskDone(token.Pass(), SYNC_STATUS_RETRY);
     return;
   }
 
   TrackerIDSet trackers;
   if (metadata_database()->GetMultiParentFileTrackers(
           &target_file_id_, &trackers)) {
     DCHECK_LT(1u, trackers.size());
     if (!trackers.has_active()) {
       NOTREACHED();
       SyncTaskManager::NotifyTaskDone(token.Pass(), SYNC_STATUS_FAILED);
@@ skipping 255 matching lines @@
   set_used_network(true);
   return sync_context_->GetDriveService();
 }
 
 MetadataDatabase* ConflictResolver::metadata_database() {
   return sync_context_->GetMetadataDatabase();
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system