Added support for EXT_frag_depth

This change also required that support be added for associating built-in
variables with an extension, similar to how functions could be associated with
extensions previously.

R=alokp@chromium.org

Review URL: https://codereview.appspot.com/9827044

git-svn-id: http://angleproject.googlecode.com/svn/trunk@2248 736b8ea6-26fd-11df-bfd4-992fa37f6226