This CL exclusively activates ScavengeTaskObserver, used by Scavenger
and atomic MinorMC, or MinorMCTaskObserver, used by concurrent
MinorMC.

Bug: v8:13012
Change-Id: I313b0cbe250828cc76d7d4b25e9abb457315112e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3871295
Commit-Queue: Leon Bettscheider <bettscheider@google.com>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82955}