- separated V8StackTraceImpl and AsyncStackTrace,
- V8Debugger owns all AsyncStackTrace and cleanup half of them when limit is reached (first created - first cleaned),
- V8StackTraceImpl, AsyncStackTrace and async-task-related tables in V8Debugger have weak reference to other async stack traces.
- async tasks are cleared with related async stacks.

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

Review-Url: https://codereview.chromium.org/2816043006
Cr-Commit-Position: refs/heads/master@{#44670}