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;
+
+// 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> {
+ 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);
+
+  // Returns the Tab associated with |web_state| if it exists or nil.
+  static Tab* GetTabForWebState(web::WebState* web_state);
+
+ private:
+  LegacyTabHelper(web::WebState* web_state, Tab* tab);
+  ~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_;
+
+  DISALLOW_COPY_AND_ASSIGN(LegacyTabHelper);
+};
+
+#endif  // IOS_CHROME_BROWSER_TABS_LEGACY_TAB_HELPER_H_