This should look suspiciously similar to tools/dm_flags.py. In fact, I
tweaked tools/dm_flags.py a bit to make it even more suspiciously similar.
I'll leave actually deduping this to future me.
I noticed we have an opportunity to make our Valgrind run of nanobench faster,
by not only making it not auto-calibrate (--loops 1) but also take only one
measurement (--samples 1). Should be 5-10x faster than the default.
BUG=skia:
Review URL: https://codereview.chromium.org/957503002
