Two bug fixes in KeepRunning

1. Fix a minor confusion between seconds and microseconds.
2. Increment the `iterations_` counter on every path where `KeepRunning` returns true: when it returns true, the caller will start a new iteration, so we must account for it.  Previously we didn't account for the last iteration.  If the number of iterations was large this had no visible effect but if it was small this resulted in timings that were significantly off.