Before this CL the main thread fetched the background GC stats and
added them to the main runtime-call-stats table. This resulted
in background GC stats showing up in the main thread trace.

This CL switches the background GC stats to use worker thread runtime-
calls-stats table. This is now consistent with other background
components of V8 suchs background compiler and parser.

Bug: v8:9508
Change-Id: Ic4c0685ded6024f78d0f22f81419fd5677202f25
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1776083Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63459}