Show Policy Managed USB Devices in Site Settings

This change updates the site settings UI for
chrome://settings/content/usbDevices to display USB devices that have
been allowed by policy. This is done by two new Polymer elements that
were added to allow chooser permissions to be able to display the site
permissions in the same way that other site permissions do. As a result
the old usb-devices Polymer element is deleted to use
chooser-exception-list and chooser-exception-list-entry.

UI Screenshot:
https://drive.google.com/file/d/16aBT31_iTAnoj19vGcfTKrOgsew32RQx

Design doc:
https://docs.google.com/document/d/1MPvsrWiVD_jAC8ELyk8njFpy6j1thfVU5aWT3TCWE8w/

Bug: 854329
Change-Id: Ic3b426f595546c9b1e558ebdb976f40dd051dbd8
Reviewed-on: https://chromium-review.googlesource.com/c/1274186
Commit-Queue: Ovidio de Jesús Ruiz-Henríquez <odejesush@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Dan Beam (slow until 1/29) <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626693}
24 files changed