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

sync/syncable/on_disk_directory_backing_store.h

Index: sync/syncable/on_disk_directory_backing_store.h
diff --git a/sync/syncable/on_disk_directory_backing_store.h b/sync/syncable/on_disk_directory_backing_store.h
index 43bc74d1b1dc2d090fd45580e15ac813ba9aef9f..9002eb04d920d6ac0977a497f8b8a92e508551f1 100644
--- a/sync/syncable/on_disk_directory_backing_store.h
+++ b/sync/syncable/on_disk_directory_backing_store.h
@@ -25,6 +25,11 @@ class SYNC_EXPORT_PRIVATE OnDiskDirectoryBackingStore
                      MetahandleSet* metahandles_to_purge,
                      Directory::KernelLoadInfo* kernel_load_info) override;
 
+ protected:
+  // Subclasses may override this to avoid a possible DCHECK.
+  virtual void ReportFirstTryOpenFailure();
+
+ private:
   // A helper function that will make one attempt to load the directory.
   // Unlike Load(), it does not attempt to recover from failure.
   DirOpenResult TryLoad(Directory::MetahandlesMap* handles_map,
@@ -32,13 +37,6 @@ class SYNC_EXPORT_PRIVATE OnDiskDirectoryBackingStore
                         MetahandleSet* metahandles_to_purge,
                         Directory::KernelLoadInfo* kernel_load_info);
 
- protected:
-  // Subclasses may override this to avoid a possible DCHECK.
-  virtual void ReportFirstTryOpenFailure();
-
- private:
-  FRIEND_TEST_ALL_PREFIXES(DirectoryBackingStoreTest, MinorCorruption);
-
   bool allow_failure_for_test_;
   base::FilePath backing_filepath_;