[SYNC] Sessions datatype refactor. Most things related to sessions under-the-...

chrome/browser/sessions/session_id.h

 // Copyright (c) 2006-2008 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 CHROME_BROWSER_SESSIONS_SESSION_ID_H_
 #define CHROME_BROWSER_SESSIONS_SESSION_ID_H_
 #pragma once
 
 #include "base/basictypes.h"
 
 class NavigationController;
 class SessionService;
+namespace browser_sync {
+  class SessionModelAssociator;
+}
 
 // Uniquely identifies a tab or window for the duration of a session.
 class SessionID {
  public:
   typedef int32 id_type;
 
   SessionID();
   ~SessionID() {}
 
   // Returns the underlying id.
   id_type id() const { return id_; }
 
  private:
   friend class NavigationController;
   friend class SessionService;
+  friend class browser_sync::SessionModelAssociator;
 
   explicit SessionID(id_type id) : id_(id) {}
 
   // Resets the id. This is used when restoring a session
   void set_id(id_type id) { id_ = id; }
 
   id_type id_;
 };
 
 #endif  // CHROME_BROWSER_SESSIONS_SESSION_ID_H_