Enable use_mojo on mac

BUG=
R=sky@chromium.org

Review URL: https://codereview.chromium.org/221463005

git-svn-id: http://src.chromium.org/svn/trunk/src/build@261202 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -864,7 +864,7 @@
           'enable_printing%': 0,
         }],
 
-        ['OS=="win" or OS=="linux"', {
+        ['OS=="win" or OS=="linux" or OS=="mac"', {
           'use_mojo%': 1,
         }],