[ios] Add a BrowserListObserver interface.

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

Index: ios/shared/chrome/browser/ui/browser_list/browser_list_observer.mm
diff --git a/ios/shared/chrome/browser/ui/browser_list/browser_list_observer.mm b/ios/shared/chrome/browser/ui/browser_list/browser_list_observer.mm
new file mode 100644
index 0000000000000000000000000000000000000000..ac6a9e472f40477e866a93a6e92b741a848764f2
--- /dev/null
+++ b/ios/shared/chrome/browser/ui/browser_list/browser_list_observer.mm
@@ -0,0 +1,19 @@
+// 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.
+
+#import "ios/shared/chrome/browser/ui/browser_list/browser_list_observer.h"
+
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
+BrowserListObserver::BrowserListObserver() = default;
+
+BrowserListObserver::~BrowserListObserver() = default;
+
+void BrowserListObserver::OnBrowserCreated(BrowserList* browser_list,
+                                           Browser* browser) {}
+
+void BrowserListObserver::OnBrowserRemoved(BrowserList* browser_list,
+                                           Browser* browser) {}