Add dependency to ICU 4.6

We use the patched version from the chromium project, as it already has
a gyp file and allows for using the system icu instead of the bundled
version if desired.

This is in preparation of bringing in v8-i18n.

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

Review URL: https://codereview.chromium.org/17850002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@15402 ce2b1a6d-e550-0410-aec6-3dcde31c8c00