The comment for maxPtrmaskBytes implied that the value was still 16,
but that changed in CL 10815.

Change-Id: I86e304bc7d9d1a0a6b22b600fefcc1325e4372d9
Reviewed-on: https://go-review.googlesource.com/36120Reviewed-by: David Crawshaw <crawshaw@golang.org>
Reviewed-by: Austin Clements <austin@google.com>