Get rid of the ability to unregister preferences.

chrome/browser/ui/prefs/prefs_tab_helper.cc

Index: chrome/browser/ui/prefs/prefs_tab_helper.cc
diff --git a/chrome/browser/ui/prefs/prefs_tab_helper.cc b/chrome/browser/ui/prefs/prefs_tab_helper.cc
index b9da3e826b5d22a5bfadb1ddf59b478f7c00f7bb..c6c9ccbfd223062aebece0ccea624b795fff9246 100644
--- a/chrome/browser/ui/prefs/prefs_tab_helper.cc
+++ b/chrome/browser/ui/prefs/prefs_tab_helper.cc
@@ -576,20 +576,18 @@ void PrefsTabHelper::RegisterUserPrefs(PrefRegistrySyncable* registry) {
   registry->RegisterStringPref(prefs::kRecentlySelectedEncoding,
                                "",
                                PrefRegistrySyncable::UNSYNCABLE_PREF);
-}
 
-void PrefsTabHelper::MigrateUserPrefs(PrefService* prefs,
-                                      PrefRegistrySyncable* registry) {
   RegisterPrefsToMigrate(registry);
+}
+
+void PrefsTabHelper::MigrateUserPrefs(PrefService* prefs) {
   for (int i = 0; i < kPrefsToMigrateLength; ++i) {
     const PrefService::Preference* pref =
         prefs->FindPreference(kPrefNamesToMigrate[i].from);
-    if (!pref) continue;
-    if (!pref->IsDefaultValue()) {
+    if (pref && !pref->IsDefaultValue()) {
       prefs->Set(kPrefNamesToMigrate[i].to, *pref->GetValue());
+      prefs->ClearPref(kPrefNamesToMigrate[i].from);

[Jói, 2013/02/20 15:34:50]

Note, I moved this here since there seems no need to look up the preference
again to clear it, unless it didn't have its default value.

     }
-    prefs->ClearPref(kPrefNamesToMigrate[i].from);
-    registry->DeprecatedUnregisterPreference(kPrefNamesToMigrate[i].from);
   }
 }