Fix CrSettingsAboutPageTest.AboutPage_OfficialBuild test.

Test was affected by paper-icon-button migration. This selects the
correct element to check for the hidden attribute.

Bug: 821759
Change-Id: I1bc67c41e9e9293c5f06ef0120fdde85beb959d6
Reviewed-on: https://chromium-review.googlesource.com/978470Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Hector Carmona <hcarmona@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545566}

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

@@ -775,7 +775,8 @@ cr.define('settings_about_page', function() {
 
           arrow = page.$$('#promoteUpdater button');
           assertTrue(!!arrow);
-          assertFalse(arrow.hidden);
+          assertEquals('PAPER-ICON-BUTTON-LIGHT', arrow.parentElement.tagName);
+          assertFalse(arrow.parentElement.hidden);
           assertFalse(arrow.hasAttribute('disabled'));
 
           firePromoteUpdaterStatusChanged(PromoStatusScenarios.IN_BETWEEN);
@@ -787,7 +788,8 @@ cr.define('settings_about_page', function() {
 
           arrow = page.$$('#promoteUpdater button');
           assertTrue(!!arrow);
-          assertFalse(arrow.hidden);
+          assertEquals('PAPER-ICON-BUTTON-LIGHT', arrow.parentElement.tagName);
+          assertFalse(arrow.parentElement.hidden);
           assertTrue(arrow.hasAttribute('disabled'));
 
           firePromoteUpdaterStatusChanged(PromoStatusScenarios.PROMOTED);
@@ -799,7 +801,8 @@ cr.define('settings_about_page', function() {
 
           arrow = page.$$('#promoteUpdater button');
           assertTrue(!!arrow);
-          assertTrue(arrow.hidden);
+          assertEquals('PAPER-ICON-BUTTON-LIGHT', arrow.parentElement.tagName);
+          assertTrue(arrow.parentElement.hidden);
           assertTrue(arrow.hasAttribute('disabled'));
         });