Add a function to fetch the installed version of Xcode.
Add a function to fetch the installed version of Xcode.

We're not using this yet, but the objectVersion in files created with 3.2 is different, so we'll probably end up needing this at some point.
Review URL: http://codereview.chromium.org/113949
old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google