Settings WebUI: Fix CrSettingsIncompatibleApplicationsPageTest with Polymer 2.

The test was failing because it was asserting on a DOM subtree that was behind a
<dom-if> that is false. In Polymer 2, a new optimization prevents those parts of
the DOM to receive any updates.

Bug: 896208
Change-Id: I268b6732ccf75f5544b3ce3011a73fbc8d11f55b
Reviewed-on: https://chromium-review.googlesource.com/c/1287199Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600622}

chrome/test/data/webui/settings/incompatible_applications_page_test.js

@@ -98,6 +98,17 @@ suite('incompatibleApplicationsHandler', function() {
    * @param {!Array<settings.IncompatibleApplication>}
    */
   function validateList(incompatibleApplications) {
+    if (incompatibleApplications.length === 0) {
+      const list =
+          incompatibleApplicationsPage.$$('#incompatible-applications-list');
+      assertEquals('none', list.style.display);
+      // The contents of a dom-if that is false no longer receive updates in
+      // Polymer 2. When there are no applications the parent dom-if becomes
+      // false, so only check that the list is hidden, but don't assert on
+      // number of DOM children.
+      return;
+    }
+
     const list = incompatibleApplicationsPage.shadowRoot.querySelectorAll(
         '.incompatible-application:not([hidden])');