Don't track representations in context extensions.
This also enables verification of representations.

BUG=
R=jkummerow@chromium.org

Review URL: https://chromiumcodereview.appspot.com/15411003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14735 ce2b1a6d-e550-0410-aec6-3dcde31c8c00