Method GCTracer::UpdateStatistics was responsible for copying
incremental and background scopes to the current event, before
reporting. It was called, however, at the end of the atomic pause and,
as a result, some of these scopes would be prematurely copied to the
current event (e.g., incremental and background sweeping scopes) and
misreported.

This CL fixes this by splitting the update of statistics and the
copying of incremental and background scopes. It introduces the
method GCTracer::FinalizeCurrentEvent which does the latter, which
is called from GCTracer::StopCycle. It also introduces methods for
correctly accessing and updating scopes, before the current event is
finalized, and eliminates the distinction between
GCTracer::AddScopeSample and GCTracer::AddScopeSampleBackground.

Bug: chromium:1154636
Change-Id: I2a6d9abb3daa2c48b2dce12dc2685cfc84130abf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3576792Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Nikolaos Papaspyrou <nikolaos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79938}