There are a few places where the integer value is used.
Use the equivalent constants to aid with readability.

Change-Id: I023b1dbe605340544c056d0e0d9d6d5a7d7d0edc
GitHub-Last-Rev: c1c90bcd251901f9f2a305ce5ddd0d85009a3d49
GitHub-Pull-Request: golang/go#24123
Reviewed-on: https://go-review.googlesource.com/96984Reviewed-by: Andrew Bonventre <andybons@golang.org>