Allow Blink embedder to decide FileSystem API permission asynchronously

This is the first of several patches to switch over the FileSystem API to be asynchronous. This CL is essentially a NOOP. It introduces the appropriate plumbing to the Blink embedder to allow an asynchronous permission check.

BUG=343382

Review URL: https://codereview.chromium.org/177003022

git-svn-id: svn://svn.chromium.org/blink/trunk@168840 bbb929c8-8fbe-4397-9dbb-9b2b20218538