[SyncFS] Refactor MetadataDatabase on dirty flag handling and sync-root creation

chrome/browser/sync_file_system/drive_backend/drive_backend_util.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/drive_backend_util.h"
 
 #include "base/file_util.h"
 #include "base/logging.h"
 #include "base/memory/scoped_vector.h"
 #include "base/message_loop/message_loop_proxy.h"
@@ skipping 12 matching lines @@
 
 void PutServiceMetadataToBatch(const ServiceMetadata& service_metadata,
                                leveldb::WriteBatch* batch) {
   std::string value;
   bool success = service_metadata.SerializeToString(&value);
   DCHECK(success);
   batch->Put(kServiceMetadataKey, value);
 }
 
 void PutFileToBatch(const FileMetadata& file, leveldb::WriteBatch* batch) {
+  if (!batch)
+    return;
+
   std::string value;
   bool success = file.SerializeToString(&value);
   DCHECK(success);
   batch->Put(kFileMetadataKeyPrefix + file.file_id(), value);
 }
 
 void PutTrackerToBatch(const FileTracker& tracker, leveldb::WriteBatch* batch) {
+  if (!batch)
+    return;
+
   std::string value;
   bool success = tracker.SerializeToString(&value);
   DCHECK(success);
   batch->Put(kFileTrackerKeyPrefix + base::Int64ToString(tracker.tracker_id()),
              value);
 }
 
 void PopulateFileDetailsByFileResource(
     const google_apis::FileResource& file_resource,
     FileDetails* details) {
@@ skipping 191 matching lines @@
 
   util::Log(logging::LOG_WARNING,
             FROM_HERE,
             "Got unexpected error: %d",
             static_cast<int>(error));
   return SYNC_STATUS_FAILED;
 }
 
 }  // namespace drive_backend
 }  // namespace sync_file_system