DevTools: Introduce Debugger::Reason::PromiseRejection pause reason.

Update the UI to display "Paused on promise rejection" status instead
of "Paused on exception" when appropriate.

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

Review URL: https://codereview.chromium.org/651533003

git-svn-id: svn://svn.chromium.org/blink/trunk@183733 bbb929c8-8fbe-4397-9dbb-9b2b20218538