With recent CLs we always store maximum N async stack traces and when we reach limit we drop half of them.
Current promise collected event requires creating weak handle:
- it takes time,
- it consumes memory.
Since async task id distribution for promises is uniform (each new promise has last_async_task_id + 1 as an id) our hash map is good enough to handle any amount of async task ids, following time of executing 1 000 000 000 of lookups:
- for empty hash map: 1.45 seconds,
- for hash map with one entry: 14.95 seconds
- 1024 entries: 15.03 seconds
- 1024 * 1024 entries: 14.82 seconds
- 1024 * 1024 * 1024: 17.9 seconds

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

Review-Url: https://codereview.chromium.org/2819423005
Cr-Commit-Position: refs/heads/master@{#44750}