Settings UI: Declare implied Polymer properties in SiteDetailsPermission

The site details page passes an icon, category, and label to each
site-details-permission element, but these properties are not declared.
As a result, in Polymer 2, the page appears distorted, and the labels
and icons are missing.

Bug: 896582
Change-Id: Ic34d8b1d49e29ce64e99ba392c62cba876b12953
Reviewed-on: https://chromium-review.googlesource.com/c/1294711Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602009}

chrome/browser/resources/settings/site_settings/site_details_permission.js

@@ -32,6 +32,10 @@ Polymer({
      * @private
      */
     defaultSetting_: String,
+
+    label: String,
+
+    icon: String,
   },
 
   observers: ['siteChanged_(site)'],