Sometimes we need to capture stack trace on one debugger and use it
later as a parent stack on another debugger (e.g. worker.postMessage).

This CL includes following addition to our protocol and v8-inspector.h:
  - added Runtime.StackTraceId, this id represents stack trace captured
    on debugger with given id,
  - protocol client can fetch Runtime.StackTrace by
    Runtime.StacKTraceId using Debugger.getStackTrace method,
  - externalParent field is added to Debugger.paused event, it may
    contain external parent stack trace,
  - V8Inspector::storeCurrentStackTrace captures current stack trace
    and returns V8StackTraceId for embedder this id can be used as
    argument for V8Inspector::externalAsyncTaskStarted and
    V8Inspector::externalAsyncTaskFinished method. Any async stack
    trace captured between these calls will get passed external stack
    trace as external parent. These methods are designed to be called
    on different debuggers. If async task is scheduled and started on
    one debugger user should continue to use asyncTask* API,
  - Debugger.enable methods returns unique debuggerId.

TBR=dgozman@chromium.org,jgruber@chromium.org

Bug: chromium:778796
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I2c1a2b2e30ed69ccb61d10f08686f4edb09f50e4
Reviewed-on: https://chromium-review.googlesource.com/786274
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Reviewed-by: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49591}