Add missing include for sleep

This fixes the AOSP mips build. Mirrors the same change needed in DM.cpp at crrev.com/1425843003

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1568883002

Review URL: https://codereview.chromium.org/1568883002
1 file changed