[ios] WebStateList owns all WebState it manages.

ios/chrome/browser/tabs/legacy_tab_helper.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_CHROME_BROWSER_TABS_LEGACY_TAB_HELPER_H_
 #define IOS_CHROME_BROWSER_TABS_LEGACY_TAB_HELPER_H_
 
-#import "base/ios/weak_nsobject.h"
+#import "base/mac/scoped_nsobject.h"
 #include "base/macros.h"
+#import "ios/web/public/web_state/web_state_observer.h"
 #import "ios/web/public/web_state/web_state_user_data.h"
 
 @class Tab;
 
-// LegacyTabHelper allows to access to the Tab owning a given WebState for
-// interoperability of code using WebStates with legacy code using Tabs.
-class LegacyTabHelper : public web::WebStateUserData<LegacyTabHelper> {
+// LegacyTabHelper associates a Tab instance to a WebState object.
+class LegacyTabHelper : public web::WebStateObserver,
+                        public web::WebStateUserData<LegacyTabHelper> {
  public:
-  // Creates the LegacyTabHelper to record the association of |web_state|
-  // with |tab|. It is an error if |web_state| is already associated with
-  // another Tab.
-  static void CreateForWebState(web::WebState* web_state, Tab* tab);
+  // Creates the LegacyTabHelper. This immediately creates the Tab object.
+  static void CreateForWebState(web::WebState* web_state);
 
-  // Returns the Tab associated with |web_state| if it exists or nil.
+  // Returns the Tab associated with |web_state| or nil.
   static Tab* GetTabForWebState(web::WebState* web_state);
 
  private:
-  LegacyTabHelper(web::WebState* web_state, Tab* tab);
+  explicit LegacyTabHelper(web::WebState* web_state);
   ~LegacyTabHelper() override;
 
-  // The Tab instance associated with the WebState. The Tab currently owns
-  // the WebState, so this should only be nil between the call to -[Tab close]
-  // and the object is deallocated.
-  base::WeakNSObject<Tab> tab_;
+  // web::WebStateObserver implementation.
+  void WebStateDestroyed() override;
+
+  // The Tab instance associated with the WebState.

[rohitrao (ping after 24h), 2017/03/26 23:21:21]

Should we document the lifetime of this Tab object?

[sdefresne, 2017/03/28 15:15:05]

Done in ios/chrome/browser/tabs/tab_lifecycle.md (in the current CL, next
patchset).

+  base::scoped_nsobject<Tab> tab_;
 
   DISALLOW_COPY_AND_ASSIGN(LegacyTabHelper);
 };
 
 #endif  // IOS_CHROME_BROWSER_TABS_LEGACY_TAB_HELPER_H_