Currently the code for completion of mark 1/mark 2 is duplicated in
background workers and assists. Factor this in to a single function
that will serve as the transition function for concurrent mark.

Change-Id: I4d9f697a15da0d349db3b34d56f3a220dd41d41b
Reviewed-on: https://go-review.googlesource.com/16359Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>