This CL moves the call to GCTracer::StopCycle for the full GC from
Heap::CompleteSweeping full, which is called to force sweeping to
finish, to GCTracer::NotifySweepingComplete, which is called as soon
as sweeping finishes --- and symmetrically to a new method
GCTracer::NotifyCppGCCompleted, which is called as soon as sweeping
of the managed C++ heap finishes. In this way, a full GC cycle is
reported as soon as sweeping is finished both for the V8 and the C++
managed heap.

The changes introduced in this CL are essentially a partial revert of
https://crrev.com/c/3456563, fixed in such a way that when the full
GC cycle is reported, the current tracer event will be the correct
one corresponding to that cycle.

Bug: v8:12503
Bug: chromium:1154636
Change-Id: Icea07cf35a9565994e798b0500e9da72cd95f9ac
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3497318Reviewed-by: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79470}