{go} sounds active, as if the state would be changed. {reached} sounds
like an accessor.

Drive-by: Make the accessor const to further clarify that this is just
an accessor.

R=titzer@chromium.org

Bug: v8:8015
Change-Id: I80d5223b9573345d3de386d68871a9e828621fc0
Reviewed-on: https://chromium-review.googlesource.com/1203992Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55614}