[Object.observe] Don't force normalization of elements for observed objects

BUG=v8:2946
R=danno@chromium.org

Review URL: https://codereview.chromium.org/29353003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17601 ce2b1a6d-e550-0410-aec6-3dcde31c8c00