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

sync/syncable/syncable_base_transaction.cc

 // Copyright 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.
 
 #include "sync/syncable/syncable_base_transaction.h"
 
 #include "base/trace_event/trace_event.h"
 #include "sync/syncable/directory.h"
 
 namespace syncer {
 namespace syncable {
 
 // static
 Id BaseTransaction::root_id() {
   return Id::GetRoot();
 }
 
 Directory* BaseTransaction::directory() const {
   return directory_;
 }
 
 void BaseTransaction::Lock() {
   TRACE_EVENT2("sync_lock_contention", "AcquireLock",
                "src_file", from_here_.file_name(),
                "src_func", from_here_.function_name());
 
-  directory_->kernel_->transaction_mutex.Acquire();
+  directory_->kernel()->transaction_mutex.Acquire();
 }
 
 void BaseTransaction::Unlock() {
-  directory_->kernel_->transaction_mutex.Release();
+  directory_->kernel()->transaction_mutex.Release();
 }
 
 void BaseTransaction::OnUnrecoverableError(
     const tracked_objects::Location& location,
     const std::string& message) {
   unrecoverable_error_set_ = true;
   unrecoverable_error_location_ = location;
   unrecoverable_error_msg_ = message;
 
   // Note: We dont call the Directory's OnUnrecoverableError method right
@@ skipping 28 matching lines @@
                      "src_file", from_here_.file_name(),
                      "src_func", from_here_.function_name());
 }
 
 BaseTransaction::~BaseTransaction() {
   TRACE_EVENT_END0("sync", name_);
 }
 
 }  // namespace syncable
 }  // namespace syncer