Put object templates of the i18n extension on the heap object.

Using function local statics doesn't work, as we need the templates per
isolate.

I'm not #ifdef'ing the definitions out, because kEmptyStringRootIndex
changes depending on whether the two additional slots are present or
not.

BUG=v8:2745
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/20299002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15888 ce2b1a6d-e550-0410-aec6-3dcde31c8c00