Revert "Temporarily disable performance.now() in the d8 shell."

This reverts commit r18529.

R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/133523003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18531 ce2b1a6d-e550-0410-aec6-3dcde31c8c00