Demo UI made with a help of WUG toolkit (DO NOT SEND TO CQ).

chrome/browser/ui/webui/demo_ui/events/events_demo.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/ui/webui/demo_ui/events/events_demo.h"
+
+EventsDemo::EventsDemo() {
+}
+
+EventsDemo::~EventsDemo() {
+}
+
+void EventsDemo::Register() {
+  SetFactory(
+      [](content::BrowserContext*)
+          -> gen::EventsDemoViewModel* { return new EventsDemo(); });
+}
+
+void EventsDemo::OnButtonClicked() {
+  GetContextEditor().SetInteger(kContextKeyEventCounter, GetEventCounter() + 1);
+  FOR_EACH_OBSERVER(Observer, observers_, EventsCounterChanged());
+}
+
+void EventsDemo::AddObserver(Observer* observer) {
+  observers_.AddObserver(observer);
+}
+
+void EventsDemo::RemoveObserver(Observer* observer) {
+  observers_.RemoveObserver(observer);
+}