A DCHECK was failing if we unwind an activation which is not the
bottom-most. This CL fixes this and adds a test for this.

R=ahaas@chromium.org
BUG=v8:5822

Change-Id: Ib69116b4c45a7b2a0d6cab97ad984dfdcda55918
Reviewed-on: https://chromium-review.googlesource.com/464788Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44332}