Add the constant to cache 'Components.utils' to modules/base.js.

At https://github.com/piroor/treestyletab/commit/ce9bd03e3c2e258714517af7e22dc0f0b0d303f1, I forgot  to replace `Cu` to `Components.utils`. As the result, it's now happen the error referring undefined variable.

This patch aims to add the new constant `Cu` for resolving its error.