Fix Mac bots because of failing readback of 565
TBR=robertphillips@google.com

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1784553003

Review URL: https://codereview.chromium.org/1784553003 .
1 file changed