Introduce WebStateList to manage a list of web::WebState.

ios/shared/chrome/browser/tabs/web_state_list_observer.h

Index: ios/shared/chrome/browser/tabs/web_state_list_observer.h
diff --git a/ios/shared/chrome/browser/tabs/web_state_list_observer.h b/ios/shared/chrome/browser/tabs/web_state_list_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..6c2e85f3aa1c9af5dfeddeb26e41720f8f2fdb81
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+++ b/ios/shared/chrome/browser/tabs/web_state_list_observer.h
@@ -0,0 +1,52 @@
+// 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_TABS_WEB_STATE_LIST_OBSERVER_H_
+#define IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_OBSERVER_H_
+
+#include "base/macros.h"
+
+class WebStateList;
+
+namespace web {
+class WebState;
+}
+
+// Interface for listening to events occurring to WebStateLists.
+class WebStateListObserver {
+ public:
+  WebStateListObserver() = default;
+  virtual ~WebStateListObserver() = default;
+
+  // Invoked after a new WebState has been added to the WebStateList at the
+  // specified index.
+  virtual void WebStateInsertedAt(WebStateList* web_state_list,
+                                  web::WebState* web_state,
+                                  int index) = 0;
+
+  // Invoked after the WebState at the specified index is moved to another
+  // index.
+  virtual void WebStateMoved(WebStateList* web_state_list,
+                             web::WebState* web_state,
+                             int from_index,
+                             int to_index) = 0;
+
+  // Invoked after the WebState at the specified index is replaced by another
+  // WebState.
+  virtual void WebStateReplacedAt(WebStateList* web_state_list,
+                                  web::WebState* old_web_state,
+                                  web::WebState* new_web_state,
+                                  int index) = 0;
+
+  // Invoked after the WebState at the specified index has been detached. The
+  // WebState is still valid but is no longer in the WebStateList.
+  virtual void WebStateDetachedAt(WebStateList* web_state_list,
+                                  web::WebState* web_state,
+                                  int index) = 0;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(WebStateListObserver);
+};
+
+#endif  // IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_OBSERVER_H_