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

chrome/browser/sync_file_system/drive_backend/drive_backend_util.cc

Index: chrome/browser/sync_file_system/drive_backend/drive_backend_util.cc
diff --git a/chrome/browser/sync_file_system/drive_backend/drive_backend_util.cc b/chrome/browser/sync_file_system/drive_backend/drive_backend_util.cc
index e879f7b05b1a10a4308cae099e5be747f8392538..bd072bd475d2b6252f30d6fa067e0e2dac8ad565 100644
--- a/chrome/browser/sync_file_system/drive_backend/drive_backend_util.cc
+++ b/chrome/browser/sync_file_system/drive_backend/drive_backend_util.cc
@@ -30,6 +30,9 @@ void PutServiceMetadataToBatch(const ServiceMetadata& service_metadata,
 }
 
 void PutFileToBatch(const FileMetadata& file, leveldb::WriteBatch* batch) {
+  if (!batch)
+    return;
+
   std::string value;
   bool success = file.SerializeToString(&value);
   DCHECK(success);
@@ -37,6 +40,9 @@ void PutFileToBatch(const FileMetadata& file, leveldb::WriteBatch* batch) {
 }
 
 void PutTrackerToBatch(const FileTracker& tracker, leveldb::WriteBatch* batch) {
+  if (!batch)
+    return;
+
   std::string value;
   bool success = tracker.SerializeToString(&value);
   DCHECK(success);