mojo filesystem: Simplify full file reading/writing.

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 bd5aba4dceebf1c57dc3442452b8499d82d7c69e..4d521aea2542c06d5aeb5e3f19c527f159b8b2b2 100644
--- a/components/filesystem/public/interfaces/directory.mojom
+++ b/components/filesystem/public/interfaces/directory.mojom
@@ -52,6 +52,12 @@ interface Directory {
   // fsync()/FlushFileBuffers().
   Flush() => (FileError error);
 
+  // Reads the contents of an entire file.
+  ReadEntireFile(string path) => (FileError error, array<uint8> data);
+
+  // Writes |data| to |path|, overwriting the file if it already exists.
+  WriteFile(string path, array<uint8> data) => (FileError error);
+
   // 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