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

sync/syncable/scoped_kernel_lock.cc

Index: sync/syncable/scoped_kernel_lock.cc
diff --git a/sync/syncable/scoped_kernel_lock.cc b/sync/syncable/scoped_kernel_lock.cc
index 4a2742035feadc2d423922da7f482fde30b4f78e..a2d75729249fb4e0d09e4750f30e60d4219d4b35 100644
--- a/sync/syncable/scoped_kernel_lock.cc
+++ b/sync/syncable/scoped_kernel_lock.cc
@@ -10,7 +10,7 @@ namespace syncer {
 namespace syncable {
 
 ScopedKernelLock::ScopedKernelLock(const Directory* dir)
-  :  scoped_lock_(dir->kernel_->mutex), dir_(const_cast<Directory*>(dir)) {
+    : scoped_lock_(dir->kernel()->mutex), dir_(dir) {
 }
 
 ScopedKernelLock::~ScopedKernelLock() {}