If async stack is longer then max depth, we add externalParent as id,
client can fetch next max depth async stacks by Debugger.getStackTrace.

R=dgozman@chromium.org

Bug: chromium:778796
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I89d461e672251f03fb239f4f16ae3b0374fce766
Reviewed-on: https://chromium-review.googlesource.com/776242
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49595}