This will allow V8 extra consumers to mark a promise as handled without
adding redundant empty onRejected handlers. This is needed by streams as
discussed in https://github.com/whatwg/streams/issues/547.

BUG=chromium:654701

Review-Url: https://codereview.chromium.org/2498143002
Cr-Commit-Position: refs/heads/master@{#41012}