Fix nanobench SKP flushes between loops

Updates SKPBench to actually flush between draw loops, as this was the
original intent of the flush logic.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1715873002

Review URL: https://codereview.chromium.org/1715873002
1 file changed