s/C++'s gmtime/POSIX's gmtime_r/

BUG=skia:5923

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4341

Change-Id: I258da9bfa98f16e1abdea0f18ae786f9df118a8e
Reviewed-on: https://skia-review.googlesource.com/4341
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Hal Canary <halcanary@google.com>
1 file changed