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

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

Index: chrome/browser/extensions/api/settings_private/settings_private_delegate_factory.cc
diff --git a/chrome/browser/extensions/api/settings_private/settings_private_delegate_factory.cc b/chrome/browser/extensions/api/settings_private/settings_private_delegate_factory.cc
index fc56bc34f9f7ffda62157d955303fba4a289bcf4..e855cac044a6812ae77d36a2434b36c2694c53d9 100644
--- a/chrome/browser/extensions/api/settings_private/settings_private_delegate_factory.cc
+++ b/chrome/browser/extensions/api/settings_private/settings_private_delegate_factory.cc
@@ -6,6 +6,7 @@
 
 #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"
@@ -35,6 +36,14 @@ SettingsPrivateDelegateFactory::SettingsPrivateDelegateFactory()
 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));