Introduce incognito preference settings.

chrome/browser/prefs/pref_member.h

Index: chrome/browser/prefs/pref_member.h
diff --git a/chrome/browser/prefs/pref_member.h b/chrome/browser/prefs/pref_member.h
index 6d1ab5a129d902509c6d32c6ec69abccd4e787ea..099511aa1c26ee7a51a3c81445cf29c27f239ddc 100644
--- a/chrome/browser/prefs/pref_member.h
+++ b/chrome/browser/prefs/pref_member.h
@@ -45,6 +45,9 @@ class PrefMemberBase : public NotificationObserver {
             NotificationObserver* observer);
 
   // See PrefMember<> for description.
+  void Destroy();
+
+  // See PrefMember<> for description.
   bool IsManaged() const;
 
   // NotificationObserver
@@ -92,6 +95,12 @@ class PrefMember : public subtle::PrefMemberBase {
     subtle::PrefMemberBase::Init(pref_name, prefs, observer);
   }
 
+  // Unsubscribes the PrefMember from the PrefService. After calling this
+  // function, the PrefMember may not be used any more.
+  void Destroy() {
+    subtle::PrefMemberBase::Destroy();
+  }
+
   // Check whether the pref is managed, i.e. controlled externally through
   // enterprise configuration management (e.g. windows group policy). Returns
   // false for unknown prefs.