Set WebUsbAllowDevicesForUrls policy to future

This change updates the WebUsbAllowDevicesForUrls policy entry in
policy_templates.json to have the future property set to true.
This prevents the policy from being in documentation and the
generated ADMX and linux.json file until the policy is visible
in Site settings.

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

Bug: 854329
Change-Id: Ib2c9a04fd992d8c6c88a52ec9c954a48ff9a673d
Reviewed-on: https://chromium-review.googlesource.com/c/1262082
Commit-Queue: Ovidio Henriquez <odejesush@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601277}

components/policy/resources/policy_templates.json

@@ -4220,6 +4220,7 @@
         }
       },
       'supported_on': ['chrome_os:71-', 'android:71-', 'chrome.*:71-'],
+      'future': True,
       'features': {
         'dynamic_refresh': True,
         'per_profile': True,