DevTools: make revealers and renderers Promise-based

R=apavlov@chromium.org, vsevik@chromium.org

Review URL: https://codereview.chromium.org/610213002

git-svn-id: svn://svn.chromium.org/blink/trunk@182842 bbb929c8-8fbe-4397-9dbb-9b2b20218538