[ios] Add a BrowserListObserver interface.

ios/shared/chrome/browser/ui/browser_list/browser_list.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_H_
 #define IOS_SHARED_CHROME_BROWSER_UI_BROWSER_LIST_BROWSER_LIST_H_
 
 #include <memory>
 #include <vector>
 
 #include "base/macros.h"
+#include "base/observer_list.h"
 #include "base/supports_user_data.h"
 #include "ios/shared/chrome/browser/ui/browser_list/browser.h"
 
+class BrowserListObserver;
+
 namespace ios {
 class ChromeBrowserState;
 }
 
 // BrowserList attaches Browsers instance to a ChromeBrowserState.
 class BrowserList : public base::SupportsUserData::Data {
  public:
   explicit BrowserList(ios::ChromeBrowserState* browser_state);
   ~BrowserList() override;
 
@@ skipping 10 matching lines @@
 
   // Returns the index of the specified Browser, or kInvalidIndex if not found.
   int GetIndexOfBrowser(const Browser* browser) const;
 
   // Creates and returns a new Browser instance.
   Browser* CreateNewBrowser();
 
   // Closes the Browser at the specified index.
   void CloseBrowserAtIndex(int index);
 
+  // Adds/removes |observer| from the list of observers.
+  void AddObserver(BrowserListObserver* observer);
+  void RemoveObserver(BrowserListObserver* observer);
+
   // Invalid index.
   static const int kInvalidIndex = -1;
 
  private:
   ios::ChromeBrowserState* browser_state_;
   std::vector<std::unique_ptr<Browser>> browsers_;
+  base::ObserverList<BrowserListObserver, true> observers_;
 
   DISALLOW_COPY_AND_ASSIGN(BrowserList);
 };
 
 #endif  // IOS_SHARED_CHROME_BROWSER_UI_BROWSER_LIST_BROWSER_LIST_H_