A concurrent sweeper thread can access the same markbit cell as the main thread
during right trimming a fixed array, resulting in a data race on a markbit cell.
Previously we checked whether we were currently marking incrementally, filtering
out this case.

The current check has the benefit of keeping live_bytes accurate (modulo other
bugs) until the sweeper starts.

BUG=chromium:576193
LOG=N

Review URL: https://codereview.chromium.org/1576853002

Cr-Commit-Position: refs/heads/master@{#33203}