Each concurrent marking task maintains task_state[i]->marked_bytes.
When a task finishes, its local counter is flushed into global
total_marked_bytes_ atomic counter.

Bug: chromium:694255
Change-Id: I629467385e80bf229e06a4231673ceb5ef8e4aea
Reviewed-on: https://chromium-review.googlesource.com/704823
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48374}