[Extensions] Clean up renderer channel-related code

chrome/browser/extensions/chrome_notification_observer.h

-// Copyright 2013 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_EXTENSIONS_CHROME_NOTIFICATION_OBSERVER_H_
-#define CHROME_BROWSER_EXTENSIONS_CHROME_NOTIFICATION_OBSERVER_H_
-
-#include "base/compiler_specific.h"
-#include "base/macros.h"
-#include "content/public/browser/notification_observer.h"
-#include "content/public/browser/notification_registrar.h"
-
-class Browser;
-
-namespace content {
-class RenderProcessHost;
-}
-
-namespace extensions {
-
-// Observer for Chrome-specific notifications that need to be relayed to the
-// extensions module.
-class ChromeNotificationObserver : public content::NotificationObserver {
- public:
-  ChromeNotificationObserver();
-  ~ChromeNotificationObserver() override;
-
-  // Notification handlers:
-  void OnRendererProcessCreated(content::RenderProcessHost* process);
-
-  // content::NotificationObserver overrides:
-  void Observe(int type,
-               const content::NotificationSource& source,
-               const content::NotificationDetails& details) override;
-
- private:
-  content::NotificationRegistrar registrar_;
-
-  DISALLOW_COPY_AND_ASSIGN(ChromeNotificationObserver);
-};
-
-}  // namespace extensions
-
-#endif  // CHROME_BROWSER_EXTENSIONS_CHROME_NOTIFICATION_OBSERVER_H_