[webui] Make FIDO owners own FIDO webui plumbing

Make FIDO owners own settings_security_key_handler.{cc/h}.

Fixed: 1152556
Change-Id: I396e2119a660d78260469845da0e18b194bc77a1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2559251
Auto-Submit: Nina Satragno <nsatragno@chromium.org>
Reviewed-by: dpapad <dpapad@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#830813}

chrome/browser/ui/webui/settings/OWNERS

@@ -8,3 +8,4 @@ per-file *site_settings*=sauski@google.com
 per-file safe_browsing_handler*=msramek@chromium.org
 per-file safe_browsing_handler*=sauski@google.com
 per-file safety_check_handler*=andzaytsev@google.com
+per-file settings_security_key_handler*=file://device/fido/OWNERS