Add support for iOS WatchKit apps in GYP

Adds new target types for Apple Watch apps and their corresponding
WatchKit extensions, along with a test and sample application.

Patch by David Patierno <dmpatierno@google.com>

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

Review URL: https://codereview.chromium.org/762673002

git-svn-id: http://gyp.googlecode.com/svn/trunk@2014 78cadc50-ecff-11dd-a971-7dbc132099af