[win] Add signin support for the app launcher.

ash/shell/app_list.cc

Index: ash/shell/app_list.cc
diff --git a/ash/shell/app_list.cc b/ash/shell/app_list.cc
index 21090c246f10e56532265392738c140d33076e85..4415370e9e6c13e5d17da1135a90cab93a2326fd 100644
--- a/ash/shell/app_list.cc
+++ b/ash/shell/app_list.cc
@@ -225,6 +225,10 @@ class ExampleAppListViewDelegate : public app_list::AppListViewDelegate {
     DecorateSearchBox(model_->search_box());
   }
 
+  virtual app_list::SigninDelegate* GetSigninDelegate() OVERRIDE {
+    return NULL;
+  }
+
   virtual void ActivateAppListItem(app_list::AppListItemModel* item,
                                    int event_flags) OVERRIDE {
     static_cast<WindowTypeLauncherItem*>(item)->Activate(event_flags);