DevTools: Improve CommandLineAPI's getEventListeners result.

Expose "type" string and "remove()" function.

BUG=309524
R=caseq, pfeldman

Review URL: https://codereview.chromium.org/48363002

git-svn-id: svn://svn.chromium.org/blink/trunk@160713 bbb929c8-8fbe-4397-9dbb-9b2b20218538