IsActive() is only supposed to be used in DCHECKs and supporting
this is going to get harder when introducing safepoints across multiple
isolates because there won't be this single counter anymore to check.
With AssertActive() we can just invoke AssertHold() on our mutex.

No functional changes.

Bug: v8:11708
Change-Id: Ic8d17738afdc90e92e6b54f615ec9757a826cc64
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3207903
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77272}