Bugfix: A TransitionArray can disappear during copy.

During handlification of TransitionArray code, an error was introduced
in TransitionArray::CopyInsert because after creating a copy of a
TransitionArray, it may be that the array disappears during GC
because it is modified during the marking of the owning map.

R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/228483005

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20654 ce2b1a6d-e550-0410-aec6-3dcde31c8c00