Add 'External storage preferences' settings page for Chrome OS

CL:1583420 added a new Chrome pref that controls ARC's storage access.
This CL adds a new UI to chrome://settings to update the pref value.

UI mock: go/filesapp-arc-extdriveaccess (page 9)

BUG=936814
TEST=try

Change-Id: I6708456460f40e7c61ddcff573eabbf8f88ede24
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1583425
Commit-Queue: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Reviewed-by: Yury Khmel <khmel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659647}
13 files changed