If protocol client needs to make step-into async call:
- pause before async call using any Debugger agent capabilities,
- call Debugger.stepInto with breakOnAsyncCall flag,
- wait for Debugger.paused event, this event will contain
  asyncCallStackTrace if async call is scheduled,
- call Debugger.pauseOnAsyncCall on each known target,
- resume execution in current debugger by Debugger.resume.

Bug: chromium:778796
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I40c56278e7b1ceafc3bf81608b8ca6716c2b3168
Reviewed-on: https://chromium-review.googlesource.com/773573
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49594}