size_t defined as unsigned long on 31-bit s390.

R=jochen@chromium.org, jfb@chromium.org
TBR=ahaas@chromium.org,bmeurer@chromium.org,yangguo@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1889623002

Cr-Commit-Position: refs/heads/master@{#35457}