Asynchronous context tracking mechanisms in Node.js need to store some
state on all promise objects. This change will allow embedders to
configure the number of internal fields on promises as is already done
for ArrayBuffers.

BUG=v8:6435

Review-Url: https://codereview.chromium.org/2889863002
Cr-Commit-Position: refs/heads/master@{#45496}