[DevTools] Speculative crash fix in currentCallFrames

Probably break can be scheduled in DevTools and then callback can be accidently called when debugger has already disabled.
Additionally, added one more assert tp check that there is no problem with debugger script compilation.

BUG=611331
R=dgozman@chromium.org

Review-Url: https://codereview.chromium.org/2176373004
Cr-Commit-Position: refs/heads/master@{#407904}
1 file changed