Settings UI: Declare implied Polymer categoryHeader property in SiteList.

This fixes missing header text in chrome://settings/content/ subpages, when
Polymer2 is turned on.

Bug: 896621
Change-Id: I2c153ac69aca9c8e0b536e7af5388e412f9bc183
Reviewed-on: https://chromium-review.googlesource.com/c/1297597Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602202}

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

@@ -27,6 +27,8 @@ Polymer({
       value: false,
     },
 
+    categoryHeader: String,
+
     /**
      * The site serving as the model for the currently open action menu.
      * @private {?SiteException}