Upon starting a new full garbage collection the global epoch is
incremented. Since EmbedderHeapTracer::TraceEpilogue is called within
the marking phase, it is too late to finalize sweeping there as it
would use the previous cppgc epoch.

This makes metric computation crash as a single V8 full GC epoch
contains two different cppgc epochs (finalizing sweeping, new GC
cycle).

This CL avoids introducing a new EmbedderHeapTracer API and instead
makes use of the fact that cppgc is known to V8's heap.

Bug: chromium:1211795
Change-Id: I69837b083c4ee1edcad9d8c1a223278f992e5758
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3085264Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#76196}