Explicitly track whether incremental marking was activated

In the gc-tracer, we check whether we're marking to figure out which
part of the mark compact we're in. If we aborted incremental marking for
whatever reason, the check fails and we might later run into trouble

BUG=none
R=hpayer@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/783453003

Cr-Commit-Position: refs/heads/master@{#25663}