Do not scan the active part of the stack in oilpan conservative garbage collections.

If threads are allowed to continue after safepointing there is a
stack marker. We should stop stack scanning at the marker in that
case. If there is no marker the thread whose thread we are iterating
will be blocked and we should scan all the way to the recorded
end stack pointer.

R=vegorov@chromium.org
BUG=

Review URL: https://codereview.chromium.org/152513006

git-svn-id: svn://svn.chromium.org/blink/trunk@166634 bbb929c8-8fbe-4397-9dbb-9b2b20218538