1) Avoid putting empty VirtualMemory objects in {owned_code_space_}.
2) After allocating more code space, the actual allocation in that space
   should also succeed. DCHECK that.
3) Remove pointless std::move.

R=ahaas@chromium.org

Bug: v8:8015
Change-Id: Ifda30d8279579ea290c6fa8e9c310f05a044b288
Reviewed-on: https://chromium-review.googlesource.com/1238721Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56156}