[Sync] Make DirectoryBackingStore's db_ private.

sync/syncable/deferred_on_disk_directory_backing_store.cc

Index: sync/syncable/deferred_on_disk_directory_backing_store.cc
diff --git a/sync/syncable/deferred_on_disk_directory_backing_store.cc b/sync/syncable/deferred_on_disk_directory_backing_store.cc
index 3a41fabe7f25eb6fcd46fe3e3564028f2d45bd86..e4f8a1ee8e707fd8fb26d23b7686ed72cb38d56d 100644
--- a/sync/syncable/deferred_on_disk_directory_backing_store.cc
+++ b/sync/syncable/deferred_on_disk_directory_backing_store.cc
@@ -38,7 +38,7 @@ bool DeferredOnDiskDirectoryBackingStore::SaveChanges(
 
     // Reopen DB on disk.
     ResetAndCreateConnection();
-    if (!db_->Open(backing_filepath_) || !InitializeTables())
+    if (!Open(backing_filepath_) || !InitializeTables())
       return false;
 
     db_is_on_disk_ = true;
@@ -54,8 +54,8 @@ DirOpenResult DeferredOnDiskDirectoryBackingStore::Load(
     Directory::KernelLoadInfo* kernel_load_info) {
   // Open an in-memory database at first to create initial sync data needed by
   // Directory.
-  CHECK(!db_->is_open());
-  if (!db_->OpenInMemory())
+  CHECK(!IsOpen());
+  if (!OpenInMemory())
     return FAILED_OPEN_DATABASE;
 
   if (!InitializeTables())