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

components/filesystem/public/interfaces/file.mojom

Index: components/filesystem/public/interfaces/file.mojom
diff --git a/components/filesystem/public/interfaces/file.mojom b/components/filesystem/public/interfaces/file.mojom
index 306b9f78fe4c0b1bbb6bac07d1bd2f07d3eddd78..d64c0f12339d2b94a34060fc73c0552f1640c24a 100644
--- a/components/filesystem/public/interfaces/file.mojom
+++ b/components/filesystem/public/interfaces/file.mojom
@@ -59,6 +59,9 @@ interface File {
   // |open_flags| argument) as well as file position.
   Dup(File& file) => (FileError error);
 
+  // Syncs data to disk.
+  Flush() => (FileError error);
+
   // Returns a handle to the file for raw access.
   AsHandle() => (FileError error, handle file_handle);
 };