mash: Add a simple, temporary preferences store.

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 d64c0f12339d2b94a34060fc73c0552f1640c24a..ec0c6fa5ead085489c1d6de30fefe448643131c2 100644
--- a/components/filesystem/public/interfaces/file.mojom
+++ b/components/filesystem/public/interfaces/file.mojom
@@ -27,6 +27,10 @@ interface File {
   Read(uint32 num_bytes_to_read, int64 offset, Whence whence)
       => (FileError error, array<uint8>? bytes_read);
 
+  // Returns the entire contents of the file as an array of bytes. The mojo
+  // equivalent of base::ReadFileToString().
+  ReadEntireFile() => (FileError error, array<uint8>? bytes_read);
+
   // Writes |bytes_to_write| to the location specified by |offset|/|whence|.
   // TODO(vtl): Clarify behavior when |num_bytes_written| is less than the size
   // of |bytes_to_write|.