Share test framework between js-perf-tests.

Also small reformat in Iterators/forof.js

R=machenbach@chromium.org

Review URL: https://codereview.chromium.org/662723006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24682 ce2b1a6d-e550-0410-aec6-3dcde31c8c00