If the user is calling SetGCPercent(-1), they intend to disable GC.
They probably don't intend to run one. If they do, they can call
runtime.GC themselves.

Change-Id: I40ef40dfc7e15193df9ff26159cd30e56b666f73
Reviewed-on: https://go-review.googlesource.com/34013
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>