Simplify GetEnumPropertyKeys and avoid trimming fixed arrays in large object space.

BUG=352070
LOG=N
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/198943002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19894 ce2b1a6d-e550-0410-aec6-3dcde31c8c00