Fix Guest Mode errors in MD Settings by using the incognito context.

chrome/browser/extensions/api/settings_private/settings_private_delegate_factory.cc

 // Copyright 2015 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/extensions/api/settings_private/settings_private_delegate_factory.h"
 
 #include "chrome/browser/browser_process.h"
 #include "chrome/browser/extensions/api/settings_private/settings_private_delegate.h"
+#include "chrome/browser/profiles/incognito_helpers.h"
 #include "chrome/browser/profiles/profile.h"
 #include "components/keyed_service/content/browser_context_dependency_manager.h"
 #include "extensions/browser/extension_system_provider.h"
 
 namespace extensions {
 
 using content::BrowserContext;
 
 // static
 SettingsPrivateDelegate* SettingsPrivateDelegateFactory::GetForBrowserContext(
     BrowserContext* browser_context) {
   return static_cast<SettingsPrivateDelegate*>(
       GetInstance()->GetServiceForBrowserContext(browser_context, true));
 }
 
 // static
 SettingsPrivateDelegateFactory* SettingsPrivateDelegateFactory::GetInstance() {
   return base::Singleton<SettingsPrivateDelegateFactory>::get();
 }
 
 SettingsPrivateDelegateFactory::SettingsPrivateDelegateFactory()
     : BrowserContextKeyedServiceFactory(
           "SettingsPrivateDelegate",
           BrowserContextDependencyManager::GetInstance()) {
 }
 
 SettingsPrivateDelegateFactory::~SettingsPrivateDelegateFactory() {
 }
 
+content::BrowserContext* SettingsPrivateDelegateFactory::GetBrowserContextToUse(
+    content::BrowserContext* context) const {
+  // Use the incognito profile when in Guest mode.
+  if (context->IsOffTheRecord())
+    return chrome::GetBrowserContextRedirectedInIncognito(context);
+  return context;
+}
+
 KeyedService* SettingsPrivateDelegateFactory::BuildServiceInstanceFor(
     content::BrowserContext* profile) const {
   return new SettingsPrivateDelegate(static_cast<Profile*>(profile));
 }
 
 }  // namespace extensions