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

sync/syncable/directory.cc

Index: sync/syncable/directory.cc
diff --git a/sync/syncable/directory.cc b/sync/syncable/directory.cc
index e603e4e1d7d2d7ba252dd56c06f4caaa91b85e58..45e0798da5aff4a941ec49270faa57e8a751558b 100644
--- a/sync/syncable/directory.cc
+++ b/sync/syncable/directory.cc
@@ -189,6 +189,7 @@ DirOpenResult Directory::OpenImpl(
   if (OPENED != result)
     return result;
 
+  DCHECK(!kernel_);
   kernel_ = new Kernel(name, info, delegate, transaction_observer);
   delete_journal_.reset(new DeleteJournal(&delete_journals));
   InitializeIndices(&tmp_handles_map);
@@ -1548,5 +1549,13 @@ void Directory::GetAttachmentIdsToUpload(BaseTransaction* trans,
                       std::back_inserter(*ids));
 }
 
+Directory::Kernel* Directory::kernel() {
+  return kernel_;
+}
+
+const Directory::Kernel* Directory::kernel() const {
+  return kernel_;
+}
+
 }  // namespace syncable
 }  // namespace syncer