Join instead of cancel to make use of the the main thread.

Also make the Join() call explicit instead of implicitly finishing
concurrency on advancing tracing form the main thread.

Bug: v8:12600
Change-Id: I60d3e82bfc2e8a3ccc2dda761a5d3eb3ac7694d1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3578855Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79910}