Changes to default apps promo per ui leads:

chrome/browser/dom_ui/shown_sections_handler.cc

Index: chrome/browser/dom_ui/shown_sections_handler.cc
diff --git a/chrome/browser/dom_ui/shown_sections_handler.cc b/chrome/browser/dom_ui/shown_sections_handler.cc
index 6a2e3ca1c7e76197bad643a4508bf2813b415036..8d82fa785e7c1537834e0119476201e978b498ae 100644
--- a/chrome/browser/dom_ui/shown_sections_handler.cc
+++ b/chrome/browser/dom_ui/shown_sections_handler.cc
@@ -47,6 +47,15 @@ int ShownSectionsHandler::GetShownSections(PrefService* prefs) {
   return prefs->GetInteger(prefs::kNTPShownSections);
 }
 
+// static
+void ShownSectionsHandler::SetShownSection(PrefService* prefs,
+                                           Section section) {
+  int shown_sections = GetShownSections(prefs);
+  shown_sections &= ~ALL_SECTIONS_MASK;
+  shown_sections |= section;
+  prefs->SetInteger(prefs::kNTPShownSections, shown_sections);
+}
+
 ShownSectionsHandler::ShownSectionsHandler(PrefService* pref_service)
     : pref_service_(pref_service) {
   pref_registrar_.Init(pref_service);