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

components/sessions/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_
-
-#include "base/basictypes.h"
-#include "components/sessions/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_