[Navigation Experiment] Add WKBasedNavigationManagerImpl.

ios/web/navigation/session_storage_builder.h

 // Copyright 2015 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_NAVIGATION_SERIALIZED_NAVIGATION_MANAGER_BUILDER_H_
 #define IOS_WEB_NAVIGATION_SERIALIZED_NAVIGATION_MANAGER_BUILDER_H_
 
 @class CRWSessionStorage;
 
 namespace web {
 
 class WebStateImpl;
 
 // Class that can serialize and deserialize session information.
 class SessionStorageBuilder {
  public:
   // Creates a serializable session storage from |web_state|.
   CRWSessionStorage* BuildStorage(WebStateImpl* web_state) const;
   // Populates |web_state| with |storage|'s session information.
+  // The provided |web_state| must already have a |NavigationManager|.
   void ExtractSessionState(WebStateImpl* web_state,
                            CRWSessionStorage* storage) const;
 };
 
 }  // namespace web
 
 #endif  // IOS_WEB_NAVIGATION_SERIALIZED_NAVIGATION_MANAGER_BUILDER_H_