Incremental marking job and concurrent marking tasks may be triggered
during heap tear down. The task scheduler may already be torn down at
this point. Avoid scheduling new tasks if tasks are disabled.

Bug: v8:7238
Change-Id: I3869a20dc8b4f38b26754872448952a9aadb6bd9
Reviewed-on: https://chromium-review.googlesource.com/868825Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#50641}