DevTools: added promise-based runtime accessors.
We are going to migrate to them entirely over time.

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

Review URL: https://codereview.chromium.org/614583002

git-svn-id: svn://svn.chromium.org/blink/trunk@182836 bbb929c8-8fbe-4397-9dbb-9b2b20218538