[ios] Add a BrowserListObserver interface.

ios/shared/chrome/browser/ui/browser_list/browser_list_observer.h

+// Copyright 2017 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 IOS_SHARED_CHROME_BROWSER_UI_BROWSER_LIST_BROWSER_LIST_OBSERVER_H_
+#define IOS_SHARED_CHROME_BROWSER_UI_BROWSER_LIST_BROWSER_LIST_OBSERVER_H_
+
+#include "base/macros.h"
+
+class Browser;
+class BrowserList;
+
+// Interface for observing modifications of a BrowserList.
+class BrowserListObserver {
+ public:
+  BrowserListObserver();
+  virtual ~BrowserListObserver();
+
+  // Invoked after |browser| has been added to |browser_list|.
+  virtual void OnBrowserCreated(BrowserList* browser_list, Browser* browser);
+
+  // Invoked before |browser| is removed from |browser_list|.
+  virtual void OnBrowserRemoved(BrowserList* browser_list, Browser* browser);
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(BrowserListObserver);
+};
+
+#endif  // IOS_SHARED_CHROME_BROWSER_UI_BROWSER_LIST_BROWSER_LIST_OBSERVER_H_