The main thread isolate may live until the end of the process, in which
case we would not record the counts. Sample at each event instead.

R=ahaas@chromium.org

Bug: v8:8091
Change-Id: I5b1eb023e4ca5410c7d8f4212ff20410044bf4f2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2710433
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72947}