[Sync] Make DirectoryBackingStore's db_ private.

sync/syncable/on_disk_directory_backing_store.cc

Index: sync/syncable/on_disk_directory_backing_store.cc
diff --git a/sync/syncable/on_disk_directory_backing_store.cc b/sync/syncable/on_disk_directory_backing_store.cc
index f369e9360bc952ff835cdf3418af128c63d7aea1..e6f284d3dd66b80332b839261913488ae95872fc 100644
--- a/sync/syncable/on_disk_directory_backing_store.cc
+++ b/sync/syncable/on_disk_directory_backing_store.cc
@@ -38,8 +38,9 @@ DirOpenResult OnDiskDirectoryBackingStore::TryLoad(
     MetahandleSet* metahandles_to_purge,
     Directory::KernelLoadInfo* kernel_load_info) {
   DCHECK(CalledOnValidThread());
-  if (!db_->is_open()) {
-    if (!db_->Open(backing_filepath_))
+
+  if (!IsOpen()) {
+    if (!Open(backing_filepath_))
       return FAILED_OPEN_DATABASE;
   }