runtime: add a non-functional memory limit to the pacer

runtime: add a non-functional memory limit to the pacer

Nothing much to see here, just some plumbing to make latter CLs smaller
and clearer.

For #48409.

Change-Id: Ide23812d5553e0b6eea5616c277d1a760afb4ed0
Reviewed-on: https://go-review.googlesource.com/c/go/+/393401
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>