This adds a counter for the time it takes per single code GC. It is
sampled in all isolates whenever a GC finishes, and during regular
JavaScript GC to also get samples for GCs that never finish (which we
would need to fix).

R=mstarzinger@chromium.org, mpearson@chromium.org

Bug: v8:8217
Change-Id: I722d01b6bd7832c3af636362acfddeab7e8a485e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1615250
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61617}