Adds reporting for
- V8.GCMarkCompactor as sum of V8 mark-compact events

Bug: chromium:843903
Change-Id: I5e8a80c8d1a9c5bf696635b54659ac56403f52d5
Reviewed-on: https://chromium-review.googlesource.com/c/1256764
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56750}