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
DevTools: follow up to r183724, mark all floating components as roots.

BUG=417275

Review URL: https://codereview.chromium.org/660463002

git-svn-id: svn://svn.chromium.org/blink/trunk@183734 bbb929c8-8fbe-4397-9dbb-9b2b20218538