DevTools: Expand protocol to allow setting DOM event breakpoints on a given event target.

This will allow debugger to pause on "load", "error" and etc. event breakpoints only
when they happen on XHR event targets.

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

Review URL: https://codereview.chromium.org/320933003

git-svn-id: svn://svn.chromium.org/blink/trunk@175882 bbb929c8-8fbe-4397-9dbb-9b2b20218538