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

sync/syncable/syncable_base_transaction.cc

Index: sync/syncable/syncable_base_transaction.cc
diff --git a/sync/syncable/syncable_base_transaction.cc b/sync/syncable/syncable_base_transaction.cc
index 8eb6d6bdf2d7d3709f0f56ee8f57b4c493f8e78e..c145c1f339c22b582e2a3d3a29f2b1fbf6de18a3 100644
--- a/sync/syncable/syncable_base_transaction.cc
+++ b/sync/syncable/syncable_base_transaction.cc
@@ -24,11 +24,11 @@ void BaseTransaction::Lock() {
                "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(