This reverts commit 20d5048a.

Revert "[heap] Ensure that concurrent marking tasks exit before heap tear down."

This reverts commit 387f65d4.

Reason: concurrent marking tasks waiting for a signal from the main thread
is susceptible to deadlocks. We should instead re-schedule concurrent marking
threads once they exit.

BUG=chromium:694255

Change-Id: I20db2f26b42e960f4cc04506d9598c1187b8a003
Reviewed-on: https://chromium-review.googlesource.com/571800
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46671}