mojo: Fix leveldb unittests by making fs::Directories cloneable.

components/filesystem/public/interfaces/directory.mojom

Index: components/filesystem/public/interfaces/directory.mojom
diff --git a/components/filesystem/public/interfaces/directory.mojom b/components/filesystem/public/interfaces/directory.mojom
index 397c0866843a9aa599aa6e564e514450be0b3d64..063fb0ad37da8dfb790637c2b9a8b738cce483cb 100644
--- a/components/filesystem/public/interfaces/directory.mojom
+++ b/components/filesystem/public/interfaces/directory.mojom
@@ -71,6 +71,9 @@ interface Directory {
   [Sync]
   StatFile(string path) => (FileError error, FileInformation? file_information);
 
+  // Creates a copy of this directory.
+  Clone(Directory& directory);
+
   // Reads the contents of an entire file.
   ReadEntireFile(string path) => (FileError error, array<uint8> data);