We are going to introduce safepoints across multiple isolates, with
this the name GlobalSafepoint might be misleading. Use IsolateSafepoint
as name to emphasise this class reaches a safepoint for a single
isolate only.

No functional changes.

Bug: v8:11708
Change-Id: I8254031dd0bc8e6dcf9f7353297803c37dba47ed
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3207901
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77268}