Fix Unhandled ReferenceError in debug-debugger.js

This fixes following exception in Sky on attempt to set a breakpoint
"Unhandled: Uncaught ReferenceError: break_point is not defined"
I think this happens in Sky but not in Chrome because Sky scripts are executed in strict mode.

BUG=None
LOG=N
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/741683002

Cr-Commit-Position: refs/heads/master@{#25415}