[Retry][SyncFS] Implement LargestKnownChangeID handling

chrome/browser/sync_file_system/drive_backend/metadata_database.h

Index: chrome/browser/sync_file_system/drive_backend/metadata_database.h
diff --git a/chrome/browser/sync_file_system/drive_backend/metadata_database.h b/chrome/browser/sync_file_system/drive_backend/metadata_database.h
index 02b44699649cf7fbcbb924a1393c5a6e32d47de8..ef459f60470bd142fd4052a772252d5ae7d24567 100644
--- a/chrome/browser/sync_file_system/drive_backend/metadata_database.h
+++ b/chrome/browser/sync_file_system/drive_backend/metadata_database.h
@@ -335,10 +335,13 @@ class MetadataDatabase {
   void WriteToDatabase(scoped_ptr<leveldb::WriteBatch> batch,
                        const SyncStatusCallback& callback);
 
+  bool HasNewerFileMetadata(const std::string& file_id, int64 change_id);
+
   scoped_refptr<base::SequencedTaskRunner> task_runner_;
   scoped_ptr<leveldb::DB> db_;
 
   scoped_ptr<ServiceMetadata> service_metadata_;
+  int64 largest_known_change_id_;
 
   FileByID file_by_id_;  // Owned.
   TrackerByID tracker_by_id_;  // Owned.