DevTools: Optimize fetching debugger stack traces.

Optimization: initialize lazily everything that does not require the debugger
being on pause (i.e. depends on a valid break_id).

This will boost the speed up to 30%!

BUG=340285
R=yurys

Review URL: https://codereview.chromium.org/185383007

git-svn-id: svn://svn.chromium.org/blink/trunk@169003 bbb929c8-8fbe-4397-9dbb-9b2b20218538