metrics_daemon: fix harmless confusion in writing persistent integers

As the subject says, I don't think this can change any of the
behavior, but the argument to Write() was ignored and merely
confusing.

BUG=chromium:339588
TEST=ran unit tests

Change-Id: Ie6d59e6879a734523978c21d66170721e0f499c7
Reviewed-on: https://chromium-review.googlesource.com/194919


Tested-by: Luigi Semenzato <semenzato@chromium.org>
Reviewed-by: Daniel Erat <derat@chromium.org>
Reviewed-by: Luigi Semenzato <semenzato@chromium.org>
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>