Make extensions DSE persistent in browser prefs (Reland)

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 b13f76d10916699789eb5cd8fdfd21d6e399bad3..81241480c904344636d0ea6efbab734d51554d0f 100644
--- a/chrome/browser/prefs/session_startup_pref.h
+++ b/chrome/browser/prefs/session_startup_pref.h
@@ -36,10 +36,12 @@ struct SessionStartupPref {
   // line up. These are the values registered in prefs.
   // The values are also recorded in Settings.StartupPageLoadSettings histogram,
   // so make sure to update histograms.xml if you change these.
-  static const int kPrefValueLast = 1;
-  static const int kPrefValueURLs = 4;
-  static const int kPrefValueNewTab = 5;
-  static const int kPrefValueMax = 6;
+  enum PrefValue {
+    kPrefValueLast = 1,
+    kPrefValueURLs = 4,
+    kPrefValueNewTab = 5,
+    kPrefValueMax = 6,
+  };
 
   static void RegisterProfilePrefs(user_prefs::PrefRegistrySyncable* registry);