Correctly handle holes when concat()ing double arrays

BUG=chromium:403409
LOG=y
R=verwaest@chromium.org

Review URL: https://codereview.chromium.org/468863003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23144 ce2b1a6d-e550-0410-aec6-3dcde31c8c00