This CL aligns the library implementation with the blink implementation:
(*) Concurrent marking increases job priority if no concurrent progress
    is made in the last 50% of the expected marking duration.
(*) Concurrent sweeping increases job priority when calling
    FinishIfRunning (the library equivalent of blink's CompleteSweep).

Bug: chromium:1056170
Change-Id: Ice275cb90a7dd76bf4125f4338d9d80e5f576c58
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2431572
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70375}