Adds swift file support to xcode generation.

BUG=
Patch from Josh Converse <jconverse@google.com>!

R=justincohen@chromium.org, thomasvl@chromium.org

Review URL: https://codereview.chromium.org/560683003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1980 78cadc50-ecff-11dd-a971-7dbc132099af
6 files changed