Test that Managed objects get collected if they are not referenced any
more.

R=ulan@chromium.org

Bug: v8:7628
Change-Id: I4a594ebe835071d76d7a6e2ddee6a6092b3e4a31
Reviewed-on: https://chromium-review.googlesource.com/999482Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52438}