increase inner-loop for xfer4 benches to reduce variance

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1653143004

TBR=

Review URL: https://codereview.chromium.org/1653143004
1 file changed