Test sk_linear_to_srgb() less exhaustively in Debug mode.

It's pretty slow when not optimized... it can take ~5 minutes to run.
This makes it take ~2 seconds on my laptop.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2221803002

Review-Url: https://codereview.chromium.org/2221803002
1 file changed