This will be used in the future to control backend concurrency.
See CL 40693.

In the meantime, make it a no-op.
This should fix the linux-amd64-racecompile builders.

Change-Id: Ibf3b2a7fff6f8f8c94f5fafb26e0500a51c8a4a6
Reviewed-on: https://go-review.googlesource.com/41614
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>