Introduce incognito preference settings.

chrome/browser/prefs/default_pref_store.h

Index: chrome/browser/prefs/default_pref_store.h
diff --git a/chrome/browser/prefs/default_pref_store.h b/chrome/browser/prefs/default_pref_store.h
index 9e2e715eb50af62fe382e7c2b26bd0b380ebbabb..97a72954d5e00b0a2b11c0b7a02d7926f5edc201 100644
--- a/chrome/browser/prefs/default_pref_store.h
+++ b/chrome/browser/prefs/default_pref_store.h
@@ -7,6 +7,7 @@
 #pragma once
 
 #include <map>
+#include <string>
 
 #include "base/basictypes.h"
 #include "chrome/browser/prefs/value_map_pref_store.h"
@@ -20,9 +21,18 @@ class DefaultPrefStore : public ValueMapPrefStore {
 
   // Stores a new |value| for |key|. Assumes ownership of |value|.
   void SetDefaultValue(const std::string& key, Value* value) {
+    CHECK(GetValue(key, NULL) == READ_NO_VALUE);
     SetValue(key, value);
   }
 
+  // Returns the registered type for |key| or Value::TYPE_NULL if the |key|
+  // has not been registered.
+  Value::ValueType GetType(const std::string& key) const {
+    Value* value;
+    return GetValue(key, &value) == READ_OK ? value->GetType()
+                                            : Value::TYPE_NULL;
+  }
+
  private:
   DISALLOW_COPY_AND_ASSIGN(DefaultPrefStore);
 };