- Completely rely on the concurrent sweeping state for SweepingCompleted()
- Rename the state accordingly.

CQ_EXTRA_TRYBOTS=tryserver.v8:v8_linux_arm64_gc_stress_dbg,v8_linux_gc_stress_dbg,v8_mac_gc_stress_dbg,v8_linux64_asan_rel,v8_linux64_tsan_rel,v8_mac64_asan_rel
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/1614953002

Cr-Commit-Position: refs/heads/master@{#33490}