Contention profiling is off by default.
If you turn it on, it has the unfortunate effect of making
the wakeup on a contention mutex go from O(1) to O(n).
Change it back to O(1).

This is already fixed in essentially the same way on master;
master also contains some fixes for the non-profiling code
paths.

Possible for Go 1.8.1.

Change-Id: Iaa644c06e20ca28da4dfa348b7211eedb657e0ba
Reviewed-on: https://go-review.googlesource.com/37341
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>