... to make AsyncFunction subclassing work.

Bug: chromium:725537
Change-Id: I7edf4891e14e01567046e7536b3aa93877111448
Reviewed-on: https://chromium-review.googlesource.com/517087
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45544}