Enable Object.observe by default

R=rossberg@chromium.org, rossberg
BUG=v8:2409
LOG=Y

Review URL: https://codereview.chromium.org/183683022

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19734 ce2b1a6d-e550-0410-aec6-3dcde31c8c00