Make the default startup pref to be NTP also on Mac.

chrome/browser/prefs/session_startup_pref.h

Index: chrome/browser/prefs/session_startup_pref.h
diff --git a/chrome/browser/prefs/session_startup_pref.h b/chrome/browser/prefs/session_startup_pref.h
index e1885aa988033ed25c69d2aa74876129e1406f89..48e2e1146a02af196330d13b5769a5a76bf1a7ff 100644
--- a/chrome/browser/prefs/session_startup_pref.h
+++ b/chrome/browser/prefs/session_startup_pref.h
@@ -58,6 +58,10 @@ struct SessionStartupPref {
   // the same effect.
   static void MigrateIfNecessary(PrefService* prefs);
 
+  // The default startup pref for Mac used to be LAST, now it's DEFAULT. This
+  // migrates old users by writing out the preference explicitly.
+  static void MigrateMacDefaultPrefIfNecessary(PrefService* prefs);
+
   // Whether the startup type and URLs are managed via policy.
   static bool TypeIsManaged(PrefService* prefs);
   static bool URLsAreManaged(PrefService* prefs);