Slim down Sk4fRoundtrip benches.

Looks like the conversions are complicated enough to not compile away.
No need to mess with random number generation, etc.
We still need to blackhole it into a volatile in the end.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1682793002

Review URL: https://codereview.chromium.org/1682793002
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