Add more per-tab preferences.

chrome/browser/prefs/default_pref_store.cc

 // Copyright (c) 2011 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.
 
 #include "chrome/browser/prefs/default_pref_store.h"
 
 using base::Value;
 
 DefaultPrefStore::DefaultPrefStore() {}
 
 DefaultPrefStore::~DefaultPrefStore() {}
 
 void DefaultPrefStore::SetDefaultValue(const std::string& key, Value* value) {
   CHECK(GetValue(key, NULL) == READ_NO_VALUE);
   SetValue(key, value);
 }
 
+void DefaultPrefStore::RemoveDefaultValue(const std::string& key) {
+  CHECK(GetValue(key, NULL) == READ_OK);
+  RemoveValue(key);
+}
+
 base::Value::Type DefaultPrefStore::GetType(const std::string& key) const {
   const Value* value;
   return GetValue(key, &value) == READ_OK ? value->GetType() : Value::TYPE_NULL;
 }