Print Preview Refresh: Fire iron resize when loading changes

Sometimes when the user is not signed in on debug builds, only ~5
destinations show up in the "Print destinations" list. Fix this by
firing iron-resize when all destinations are done loading.

Bug: None
Change-Id: Id406b8aaaeccffd3a6419ea30a3396fc6ca7d157
Reviewed-on: https://chromium-review.googlesource.com/c/1282034Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600112}

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

@@ -27,6 +27,7 @@ Polymer({
     loadingDestinations: {
       type: Boolean,
       value: false,
+      observer: 'forceIronResize',
     },
 
     listName: String,