This rename better reflects that heap_base is both used in cppgc but
also V8's GC.

Bug: v8:12691
Change-Id: Ia18ecba462d1b55cee6722452ceb28b25490a066
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3506374Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79398}