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

chrome/browser/ui/webui/demo_ui/events/events_demo.h

Index: chrome/browser/ui/webui/demo_ui/events/events_demo.h
diff --git a/chrome/browser/ui/webui/demo_ui/events/events_demo.h b/chrome/browser/ui/webui/demo_ui/events/events_demo.h
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index 0000000000000000000000000000000000000000..292fe5da13fee8a74f72da95b65b46253a04b902
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+++ b/chrome/browser/ui/webui/demo_ui/events/events_demo.h
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+// 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.
+
+#ifndef CHROME_BROWSER_UI_WEBUI_DEMO_UI_EVENTS_EVENTS_DEMO_H_
+#define CHROME_BROWSER_UI_WEBUI_DEMO_UI_EVENTS_EVENTS_DEMO_H_
+
+#include "base/macros.h"
+#include "base/observer_list.h"
+#include "components/demo_ui/events/events_demo_view_model.h"
+
+class EventsDemo : public gen::EventsDemoViewModel {
+ public:
+  class Observer {
+   public:
+    virtual void EventsCounterChanged() = 0;
+  };
+
+  EventsDemo();
+  ~EventsDemo() override;
+
+  void AddObserver(Observer* observer);
+  void RemoveObserver(Observer* observer);
+
+  static void Register();
+
+ private:
+  void OnButtonClicked() override;
+
+  ObserverList<Observer> observers_;
+
+  DISALLOW_COPY_AND_ASSIGN(EventsDemo);
+};
+
+#endif  // CHROME_BROWSER_UI_WEBUI_DEMO_UI_EVENTS_EVENTS_DEMO_H_