Wsign-compare was recently added which exposed this
issue. Using the correct type fixes the error.

R=joransiu@ca.ibm.com, jyan@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Review-Url: https://codereview.chromium.org/2528433003
Cr-Commit-Position: refs/heads/master@{#41196}