We were notifying the debug event listeners after every 1000 steps.
This CL fixes this to only notify them if we actually paused because of
a hit breakpoint.

R=ahaas@chromium.org
BUG=v8:5822

Change-Id: I00e36b89307c7e761ceb24ccdb3157056cfb8178
Reviewed-on: https://chromium-review.googlesource.com/459480Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44175}