Move all core files in //components/sessions into core/ subdir

components/sessions/core/session_id.h

 // Copyright 2014 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 COMPONENTS_SESSIONS_SESSION_ID_H_
-#define COMPONENTS_SESSIONS_SESSION_ID_H_
+#ifndef COMPONENTS_SESSIONS_CORE_SESSION_ID_H_
+#define COMPONENTS_SESSIONS_CORE_SESSION_ID_H_
 
 #include "base/basictypes.h"
-#include "components/sessions/sessions_export.h"
+#include "components/sessions/core/sessions_export.h"
 
 // Uniquely identifies a tab or window for the duration of a session.
 class SESSIONS_EXPORT SessionID {
  public:
   typedef int32 id_type;
 
   SessionID();
   ~SessionID() {}
 
   // Returns the underlying id.
   void set_id(id_type id) { id_ = id; }
   id_type id() const { return id_; }
 
  private:
   id_type id_;
 };
 
-#endif  // COMPONENTS_SESSIONS_SESSION_ID_H_
+#endif  // COMPONENTS_SESSIONS_CORE_SESSION_ID_H_