Give a CFRunLoop to the IO message loop type on Mac OS X.

base/base.gyp

Index: base/base.gyp
===================================================================
--- base/base.gyp	(revision 90809)
+++ base/base.gyp	(working copy)
@@ -153,6 +153,7 @@
         'message_loop_unittest.cc',
         'message_pump_glib_unittest.cc',
         'message_pump_libevent_unittest.cc',
+        'message_pump_mac_unittest.cc',
         'metrics/field_trial_unittest.cc',
         'metrics/histogram_unittest.cc',
         'metrics/stats_table_unittest.cc',
@@ -291,6 +292,15 @@
             'win/win_util_unittest.cc',
           ],
         }],
+        ['OS == "mac"', {
+          'sources!': [
+            'message_pump_libevent_unittest.cc',
+          ],
+        }, {  # OS != "mac"
+          'sources!': [
+            'message_pump_mac_unittest.cc',
+          ],
+        }],
       ],
     },
     {