Add mechanism to get the Tab from the associated WebState.

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"
+#include "base/macros.h"
+#import "ios/web/public/web_state/web_state_user_data.h"
+
+@class Tab;
+
+class LegacyTabHelper : public web::WebStateUserData<LegacyTabHelper> {

[rohitrao (ping after 24h), 2017/02/08 14:07:17]

Class could use a comment explaining what it does and that it's only needed for
migration.

Please add comments to the getter and ivars too.

[sdefresne, 2017/02/08 14:45:34]

Done.

+ public:
+  static void CreateForWebState(web::WebState* web_state, Tab* tab);
+
+  Tab* tab() const { return tab_.get(); }
+
+ private:
+  LegacyTabHelper(web::WebState* web_state, Tab* tab);
+  ~LegacyTabHelper() override;
+
+  base::WeakNSObject<Tab> tab_;
+
+  DISALLOW_COPY_AND_ASSIGN(LegacyTabHelper);
+};
+
+#endif  // IOS_CHROME_BROWSER_TABS_LEGACY_TAB_HELPER_H_