harmony-scoping: Allow 'const' iteration variables in strict mode.

R=rossberg@chromium.org
BUG=v8:2506
LOG=N

Committed: https://code.google.com/p/v8/source/detail?r=24834

Review URL: https://codereview.chromium.org/671913002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24842 ce2b1a6d-e550-0410-aec6-3dcde31c8c00