Enable WebUsbAllowDevicesForUrls for desktop

This change officially enables support for WebUsbAllowDevicesForUrls for
desktop. Android support is still pending on the appropriate UI changes
to properly display the permissions.

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

Bug: 854329
Change-Id: I3b05598fe1f68960ae9c0d25902342b19c294129
Reviewed-on: https://chromium-review.googlesource.com/c/1473546
Commit-Queue: Ovidio de Jesús Ruiz-Henríquez <odejesush@chromium.org>
Reviewed-by: Pavol Marko <pmarko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634275}
4 files changed