Fix signed/unsigned missmatch on 32-bit builds. BUG=angle:681 Change-Id: I6144359acd1611a13b0a582d3a31019138bb941c Reviewed-on: https://chromium-review.googlesource.com/239844 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>