Trim the fat off SkPMFloat bench.

This bench was ~75% overhead, ~25% good bench.  It is now just about the
opposite: about 30% of the runtime is loop and random number overhead, and
about 70% of the time is spent doing SkPMColor <-> SkPMFloat work.

BUG=skia:

NOPRESUBMIT=true

Review URL: https://codereview.chromium.org/968133005
1 file changed