[Sync] Eliminate friends from Directory by exposing kernel via accessor

sync/syncable/scoped_kernel_lock.h

 // Copyright (c) 2012 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 SYNC_SYNCABLE_SCOPED_KERNEL_LOCK_H_
 #define SYNC_SYNCABLE_SCOPED_KERNEL_LOCK_H_
 
 #include "base/basictypes.h"
 #include "base/synchronization/lock.h"
 
 namespace syncer {
 namespace syncable {
 
 class Directory;
 
 class ScopedKernelLock {
  public:
   explicit ScopedKernelLock(const Directory* dir);
   ~ScopedKernelLock();
 
   base::AutoLock scoped_lock_;
-  Directory* const dir_;
+  const Directory* const dir_;
   DISALLOW_COPY_AND_ASSIGN(ScopedKernelLock);
 };
 
 }  // namespace syncable
 }  // namespace syncer
 
 #endif  // SYNC_SYNCABLE_SCOPED_KERNEL_LOCK_H_