Commit 59877bfa renamed bitMarked to bitScan, since the bitmap is no
longer used for marking. However, there were several other references
to this strewn about comments and in some other constant names. Fix
these up, too.

Change-Id: I4183d28c6b01977f1d75a99ad55b150f2211772d
Reviewed-on: https://go-review.googlesource.com/28450
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>