Add performance.now() to the d8 shell.

TEST=mjsunit/d8-performance-now
R=hpayer@chromium.org

Committed: https://code.google.com/p/v8/source/detail?r=17366

Committed: https://code.google.com/p/v8/source/detail?r=17368

Review URL: https://codereview.chromium.org/32433010

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17375 ce2b1a6d-e550-0410-aec6-3dcde31c8c00