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This adds the necessary logic using some placeholder UI. The logic reads
the directory contents and allows files whose MIME type matches the
chosen file to be cycled through.

The SWA config for chrome://media-app is updated to set
include_launch_directory = true. This guarantees that at least 2
handles will now be passed to the launchQueue consumer: a folder, and
a file in that folder (then possibly more files which we ignore for now).

web_app_file_handling.externs.js is updated with definitions for the new
parts of the NativeFileSystem API that we need for directory access. This
uses `AsyncIterable`, which requires ES2018 to properly use it. Update
our closure compile settings to accept `for await`.

Bug: 1030988, b/144865794
Change-Id: I1f2185374fbd0c691a3782b430cc2cbd70027823
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2034389Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Commit-Queue: Trent Apted <tapted@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738811}