Add a comment and DCHECK to require |session_storage| to be non-nil in WebState::CreateWithStorageS…

ios/web/public/web_state/web_state.h

 // Copyright 2013 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_WEB_PUBLIC_WEB_STATE_WEB_STATE_H_
 #define IOS_WEB_PUBLIC_WEB_STATE_WEB_STATE_H_
 
 #include <stdint.h>
 
 #include <string>
@@ skipping 75 matching lines @@
     // The transition type of navigation.
     ui::PageTransition transition;
 
     // Whether this navigation is initiated by the renderer process.
     bool is_renderer_initiated;
   };
 
   // Creates a new WebState.
   static std::unique_ptr<WebState> Create(const CreateParams& params);
 
-  // Creates a new WebState from a serialized NavigationManager.
+  // Creates a new WebState from a serialized representation of the session.
+  // |session_storage| must not be nil.
   static std::unique_ptr<WebState> CreateWithStorageSession(
       const CreateParams& params,
       CRWSessionStorage* session_storage);
 
   ~WebState() override {}
 
   // Gets/Sets the delegate.
   virtual WebStateDelegate* GetDelegate() = 0;
   virtual void SetDelegate(WebStateDelegate* delegate) = 0;
 
@@ skipping 168 matching lines @@
 
   // Returns a WeakPtr<WebState> to the current WebState. Must remain private
   // and only call must be in WebStateWeakPtrFactory. Please consult that class
   // for more details. Remove as part of http://crbug.com/556736.
   virtual base::WeakPtr<WebState> AsWeakPtr() = 0;
 };
 
 }  // namespace web
 
 #endif  // IOS_WEB_PUBLIC_WEB_STATE_WEB_STATE_H_