GCC on S390 31-bit treats size_t as 'long unsigned int', which
is incompatible with %d format specifier that expects an 'int'.
Using the appropriate macro (PRIuS) instead.

R=mlippautz@chromium.org, ulan@chromium.org
BUG=

Review-Url: https://codereview.chromium.org/2412563003
Cr-Commit-Position: refs/heads/master@{#40276}