DevTools: Support XHR event listener breakpoints on frontend.

BUG=381470
R=yurys@chromium.org, pfeldman@chromium.org, vsevik, yurys

Review URL: https://codereview.chromium.org/321113005

git-svn-id: svn://svn.chromium.org/blink/trunk@176318 bbb929c8-8fbe-4397-9dbb-9b2b20218538