It is preparation step for step-into-worker. There are few changes:
- added breakOnAsyncCall flag for Debugger.stepInto. When flag is set
  and async task is scheduled before step-into finished, we pause
  execution with additional Debugger.paused event. This event contains
  additional scheduledAsyncTaskId field.
- added Debugger.pauseOnAsyncTask. This method will pause execution as
  soon as given async task is started.

This mechanism is replacement for Debugger.scheduleStepIntoAsync which
can not be used between multiple targets.

As result we can split async task scheduling in one target and
requesting break for this async task running in another target.

R=pfeldman@chromium.org

Bug: chromium:778796
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I77be0c880d91253d333c54a23a4c084e7b8549e9
Reviewed-on: https://chromium-review.googlesource.com/750071Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Pavel Feldman <pfeldman@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49127}