DevTools: Support media event listener breakpoints in frontend.

This will allow to set breakpoint on media event listeners for video and audio elements.

R=yurys, vsevik

Review URL: https://codereview.chromium.org/343653007

git-svn-id: svn://svn.chromium.org/blink/trunk@177171 bbb929c8-8fbe-4397-9dbb-9b2b20218538