Just ensure that all background and foreground tasks finished, then we
should be in a defined state

BUG=v8:6069
R=rmcilroy@chromium.org

Change-Id: Ie5bd11c61402dccb2c65cb8fe57fd1c0f550e9a7
Reviewed-on: https://chromium-review.googlesource.com/456418Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43926}