Print Preview Componentization: Add destination list items to tab order

Ensure user can tab to destination list items in the destinations
dialog.

Bug: 859846
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: I137fc8cc73ee6bfef197607ed7a75d711002a6f8
Reviewed-on: https://chromium-review.googlesource.com/1125279
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#572489}

chrome/browser/resources/print_preview/new/destination_list_item.js

@@ -50,6 +50,10 @@ Polymer({
     // </if>
   },
 
+  hostAttributes: {
+    tabindex: 0,
+  },
+
   observers: [
     'onDestinationPropertiesChange_(' +
         'destination.displayName, destination.isOfflineOrInvalid, ' +