Previously the API allowed the embedder to re fulfill a non pending
promise. This was changed as part of
c0412961.

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I53dc028fecdcc6ab05c05cfc7795e89519ed9633
Reviewed-on: https://chromium-review.googlesource.com/932968
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51536}