DevTools: Backward compatibility fix: enable DOMDebugger when an extension asks for an event listener breakpoint.

https://stackoverflow.com/questions/25764336/chrome-extension-domdebugger-api-does-not-work-anymore

BUG=412748

Review URL: https://codereview.chromium.org/590433002

git-svn-id: svn://svn.chromium.org/blink/trunk@182503 bbb929c8-8fbe-4397-9dbb-9b2b20218538