Async generators didn't correctly handle the situation where one calls
.return on a suspended-at-start async generator and passes a
promise-like object whose awaiting causes a new request to the
generator.

Bug: chromium:805729
Change-Id: I4da13ab5bd97f8c2a2c5373242a2d5e2ab0f7f10
Reviewed-on: https://chromium-review.googlesource.com/891231Reviewed-by: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50974}