We should not report promise created for async function as candidate
for stepping. Regular StepInto works fine in this case.

TBR=dgozman@chromium.org

Bug: none
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I8dafec5417df0de593cb2a1c06d6a11093e7c64b
Reviewed-on: https://chromium-review.googlesource.com/828024Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50176}