ash: Add a unit test for the system update tray item

ash/test/test_system_tray_delegate.cc

Index: ash/test/test_system_tray_delegate.cc
diff --git a/ash/test/test_system_tray_delegate.cc b/ash/test/test_system_tray_delegate.cc
index 0d5ef0e39fd143451eacec5bd503550d54729d02..9c06c0627b6e73443a645db1b432ade04f2556d0 100644
--- a/ash/test/test_system_tray_delegate.cc
+++ b/ash/test/test_system_tray_delegate.cc
@@ -17,6 +17,7 @@ namespace test {
 
 namespace {
 
+bool g_system_update_required = false;
 LoginStatus g_initial_status = LoginStatus::USER;
 
 }  // namespace
@@ -35,6 +36,11 @@ void TestSystemTrayDelegate::SetInitialLoginStatus(LoginStatus login_status) {
   g_initial_status = login_status;
 }
 
+// static
+void TestSystemTrayDelegate::SetSystemUpdateRequired(bool required) {
+  g_system_update_required = required;
+}
+
 void TestSystemTrayDelegate::SetLoginStatus(LoginStatus login_status) {
   login_status_ = login_status;
   Shell::GetInstance()->UpdateAfterLoginStatusChange(login_status);
@@ -73,6 +79,13 @@ bool TestSystemTrayDelegate::IsUserSupervised() const {
   return login_status_ == LoginStatus::SUPERVISED;
 }
 
+void TestSystemTrayDelegate::GetSystemUpdateInfo(UpdateInfo* info) const {
+  DCHECK(info);
+  info->severity = UpdateInfo::UPDATE_NORMAL;
+  info->update_required = g_system_update_required;
+  info->factory_reset_required = false;
+}
+
 bool TestSystemTrayDelegate::ShouldShowDisplayNotification() {
   return should_show_display_notification_;
 }