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: https://angleproject.googlecode.com/svn/trunk@2248 736b8ea6-26fd-11df-bfd4-992fa37f6226 TRAC #23333 Authored-by: bajones@chromium.org Signed-off-by: Shannon Woods Signed-off-by Nicolas Capens Merged-by: Jamie Madill