runtime: specify behavior of SetMutexProfileFraction for negative values
runtime: specify behavior of SetMutexProfileFraction for negative values

Change-Id: Ie4da1a515d5405140d742bdcd55f54a73a7f71fe
Reviewed-on: https://go-review.googlesource.com/108175
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

akutz/go,josharian/go,cloudflare/go,cloudflare/go,akutz/go,josharian/go,josharian/go,nawawi/go,akutz/go,cloudflare/go,nawawi/go,CAFxX/go,josharian/go,golang/go,golang/go,josharian/go,akutz/go,cloudflare/go,golang/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,golang/go,akutz/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,akutz/go,nawawi/go,akutz/go,golang/go,CAFxX/go,golang/go,cloudflare/go,CAFxX/go,nawawi/go,akutz/go,golang/go,CAFxX/go,josharian/go,josharian/go,josharian/go,cloudflare/go,cloudflare/go,CAFxX/go,CAFxX/go