Add tools/nanobench_flags.py.

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
3 files changed