Flags will be protected from updates after V8 initialization (in the
future). This CL avoids any updates of the --gc-interval flag during
runtime, and instead updates a static field on the HeapAllocator
directly.

R=mlippautz@chromium.org

Bug: v8:12887
Change-Id: I17a495cae50a46d59a8159c6ece1558d4d61b949
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3687691
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80998}