Experimenting with incrementing live_byte_count_ using a relaxed CAS
operation. If no regression is found, we could get away with background
threads increasing that counter directly, instead of using separate
counters like concurrent markers.

Bug: v8:10315
Change-Id: I2e7a1f941a728f59d6e1fbd686d2eeb01ea4378a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2201765Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Dominik Inführ <dinfuehr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67809}