monobench: usability tweaks

 - Allow an second argument to limit the number of samples.
 - If no benchmarks match, warn and exit instead of infinitely looping.

The default limit of 2147483647 10ms samples will run for 9 months, which I think is long enough to not need any special infinity logic.
BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2747

Change-Id: Id70cf77b624e19dc04e1d75a71385aee3c988a80
Reviewed-on: https://skia-review.googlesource.com/2747
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed