Fix formatting specifier for MSVC++ 2013 compatibility.
Fix formatting specifier for MSVC++ 2013 compatibility.

The use of %zu in a printf format string is non-standard.  This change
is a follow-up to https://codereview.chromium.org/1417873007 to use the
standard PRIuS macro instead.

BUG=519022
TBR=brucedawson@chromium.org

Review URL: https://codereview.chromium.org/1411663012

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#357582}

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