Currently, to find out a Promise's status and result, one has to use the
debug context. This is for example done in Node.js. This new API is a
better replacement, also in the context of the debug context being
deprecated eventually.

R=franzih@chromium.org, gsathya@chromium.org, jochen@chromium.org
BUG=v8:5764

Review-Url: https://codereview.chromium.org/2589113002
Cr-Commit-Position: refs/heads/master@{#41855}