Fix wrong sysfs pathname.

This bug was introduced while adding test code
which was supposed to improve the reliability
of this program.

BUG=chromium:238890
TEST=verified that about:histograms contains samples

Change-Id: I66323292f9261f5715760d7c884f91aca1f7e453
Reviewed-on: https://gerrit.chromium.org/gerrit/57501


Reviewed-by: Sonny Rao <sonnyrao@chromium.org>
Tested-by: Luigi Semenzato <semenzato@chromium.org>
Commit-Queue: Luigi Semenzato <semenzato@chromium.org>