- we should always set creation async stack if it's available regardless existing of current parent async stack,
- we should cleanup parent link iff there is no creation and schedule async stack for parent.

Let's consider example: Promise.resolve().then(x => x).then(x => x), there is three promises which will call following instrumentation:
1) created #1 (Promise.resolve()) - collected stack #1
2) scheduled #1 - collected stack #2
3) created #2 with #1 as parent (first .then) - collected stack #3
4) created #3 with #2 as parent (first .then) - collected stack #4
5) started #2 - use stack #2 as scheduled
6) scheduled #2 - collected stack #6
7) finished #2
8) started #3 - use stack #6 as scheduled
9) scheduled #3 - collected stack #7
10) finished #3

If we collect stacks between step 4 and 5, it's possible to collect scheduled stack #2 but still have creation stack for #2 - stack #3 - so we always need to add creation event if scheduled is collected.

If we collect stacks between created and scheduled we should not remove parent link even if parent was not scheduled yet.

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

Review-Url: https://codereview.chromium.org/2844753002
Cr-Commit-Position: refs/heads/master@{#44990}