Creation stack trace points to the place where callback was actually chained, scheduled points where parent promise was resolved.
For async tasks without creation stack (e.g. setTimeout) we continue to use scheduled as creation since usually they are the same.

BUG=v8:6189
R=dgozman@chromium.org

Review-Url: https://codereview.chromium.org/2868493002
Cr-Commit-Position: refs/heads/master@{#45198}