mandoline filesystem: add a sqlite3 vfs to proxy filesystem usage.

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 5d3a1db4a1c89d311808621b45fb8678f75eec1d..daa9bbffb2a8ceddaa5d7b06c0396aa39f4dcccd 100644
--- a/components/filesystem/public/interfaces/directory.mojom
+++ b/components/filesystem/public/interfaces/directory.mojom
@@ -42,6 +42,12 @@ interface Directory {
   // |kDeleteFlag...| for details).
   Delete(string path, uint32 delete_flags) => (FileError error);
 
+  // Returns true if |path| exists.
+  Exists(string path) => (FileError error, bool exists);
+
+  // Returns true if |path| is writable.
+  IsWritable(string path) => (FileError error, bool is_writable);
+
   // TODO(vtl): directory "streaming"?
   // TODO(vtl): "make root" (i.e., prevent cd-ing, etc., to parent); note that
   // this would require a much more complicated implementation (e.g., it needs