valid fixed array, iff a gc occurred while allocating a fixed array as
part of array construction. Heap verification needs protection against
examining the elements object in this case.

R=svenpanne@chromium.org

Review URL: https://codereview.chromium.org/43383004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@17397 ce2b1a6d-e550-0410-aec6-3dcde31c8c00