The CL uses the different scheme to enable the generational barrier. The
separate global counter (is_enabled_) keeps track of the number of heaps
that enable generational GC. If at least one of the heaps enables the
generational GC, the counter will enable the write barrier. Technically,
the counter could be merged with WriteBarrier::is_enabled_, but having a
separate variable allows us to keep DCHECKs if generational barrier is
enabled.

Bug: chromium:1029379
Change-Id: Iafaa76f96acb18a73f8bde7231434e68c04cb683
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3616518Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80281}