Async stack trace can contain empty syncrhonous stack and external
stack.

R=dgozman@chromium.org

Bug: chromium:790567
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I2b04743f7c4f15a038eb1041cc7fc117d438b6b2
Reviewed-on: https://chromium-review.googlesource.com/822971
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50117}