Make nanobench --outResultsFile only work with Release builds.

It is very, very misleading to report performance numbers for Debug builds.
This makes it impossible for us to upload them to perf.skia.org.

BUG=skia:4828
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1606323002

Review URL: https://codereview.chromium.org/1606323002
1 file changed