message: set sync_needed_ as false after Sync()
diff: @@ -265,6 +265,7 @@ class S3WritableFile : public WritableFile {
  return errors::Unknown(putObjectOutcome.GetError().GetExceptionName(),
  ": ", putObjectOutcome.GetError().GetMessage());
  }
  + sync_needed_ = false;
  return Status::OK();
  }
  