DevTools: Make sure debug listener is added before any other calls into debugger.

This is needed to make sure we do not load v8 debugger multiple times.

BUG=386969
R=pfeldman@chromium.org, yurys@chromium.org, pfeldman, yurys

Review URL: https://codereview.chromium.org/343653008

git-svn-id: svn://svn.chromium.org/blink/trunk@176987 bbb929c8-8fbe-4397-9dbb-9b2b20218538