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

ios/shared/chrome/browser/tabs/web_state_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_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 when a new WebState has been added to the WebStateList at the
+  // specified index.
+  virtual void WebStateInsertedAt(WebStateList* web_state_list,
+                                  web::WebState* web_state,
+                                  size_t index) = 0;
+
+  // Invoked when the WebState at the specified index is moved to another index.

[rohitrao (ping after 24h), 2017/02/13 20:54:29]

Is this called before or after the move?  Is that an implementation detail or
does it belong in the API contract?

[sdefresne, 2017/02/14 13:39:57]

After, I've updated the documentation.

+  virtual void WebStateMoved(WebStateList* web_state_list,
+                             web::WebState* web_state,
+                             size_t from_index,
+                             size_t to_index) = 0;
+
+  // Invoked when the WebState at the specified index is replaced by another

[rohitrao (ping after 24h), 2017/02/13 20:54:29]

Is this called before or after the replacement?  Is that an implementation
detail or does it belong in the API contract?

[sdefresne, 2017/02/14 13:39:57]

After, I've updated the documentation.

+  // WebState.
+  virtual void WebStateReplacedAt(WebStateList* web_state_list,
+                                  web::WebState* old_web_state,
+                                  web::WebState* new_web_state,
+                                  size_t index) = 0;
+
+  // Invoked when the WebState at the specified index is being removed. The
+  // WebState is still valid but is no longer in the WebStateList.
+  virtual void WebStateRemovedAt(WebStateList* web_state_list,

[rohitrao (ping after 24h), 2017/02/13 20:54:29]

Is there value in calling this "Detached" instead of "Removed", for symmetry
with desktop?

[sdefresne, 2017/02/14 13:39:57]

Done.

+                                 web::WebState* web_state,
+                                 size_t index) = 0;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(WebStateListObserver);
+};
+
+#endif  // IOS_SHARED_CHROME_BROWSER_TABS_WEB_STATE_LIST_OBSERVER_H_