Move InitializeICU() to the V8 API and use it.

I can't get rid of the enable_i18n flag yet, as we need to be able to
turn off all extensions for creating the snapshot.

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

Review URL: https://codereview.chromium.org/18860007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15618 ce2b1a6d-e550-0410-aec6-3dcde31c8c00