runtime: account mark worker time before gcMarkDone

Currently gcMarkDone takes basically no time, so it's okay to account
the worker time after calling it. However, gcMarkDone is about to take
potentially *much* longer because it may perform all of mark
termination. Prepare for this by swapping the order so we account the
time before calling gcMarkDone.

Change-Id: I90c7df68192acfc4fd02a7254dae739dda4e2fcb
Reviewed-on: https://go-review.googlesource.com/16390


Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>