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

chrome/browser/prefs/session_startup_pref.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_PREFS_SESSION_STARTUP_PREF_H__
 #define CHROME_BROWSER_PREFS_SESSION_STARTUP_PREF_H__
 #pragma once
 
 #include <vector>
 
@@ skipping 40 matching lines @@
   static void SetStartupPref(PrefService* prefs,
                              const SessionStartupPref& pref);
   static SessionStartupPref GetStartupPref(Profile* profile);
   static SessionStartupPref GetStartupPref(PrefService* prefs);
 
   // If the user had the "restore on startup" property set to the deprecated
   // "Open the home page" value, this migrates them to a value that will have
   // 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);
 
   // Whether the startup type has not been overridden from its default.
   static bool TypeIsDefault(PrefService* prefs);
 
   // Converts an integer pref value to a SessionStartupPref::Type.
   static SessionStartupPref::Type PrefValueToType(int pref_value);
 
   // Returns |true| if a change to startup type or URLS was detected by
   // ProtectorService.
   static bool DidStartupPrefChange(Profile* profile);
 
   // Returns the protected backup of startup type and URLS.
   static SessionStartupPref GetStartupPrefBackup(Profile* profile);
 
   explicit SessionStartupPref(Type type);
 
   ~SessionStartupPref();
 
   // What to do on startup.
   Type type;
 
   // The URLs to restore. Only used if type == URLS.
   std::vector<GURL> urls;
 };
 
 #endif  // CHROME_BROWSER_PREFS_SESSION_STARTUP_PREF_H__