This changes the DCHECK (which could correctly fail) to be part of the
conditional that checks if we're in an async function.

Bug: chromium:751789
Change-Id: I3b8c1239ac93190055622c41fa1122e83b69d255
Reviewed-on: https://chromium-review.googlesource.com/607356Reviewed-by: Daniel Ehrenberg <littledan@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47261}