Extensions Web UI: Restore polyfill test

Fix the test to pass on Mac 10.13.

Bug: 925517
Change-Id: Ifc630605dfd8de124a3d4af77d35f4374ca0c316
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1597910Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#657433}

chrome/test/data/webui/extensions/cr_extensions_browsertest.js

@@ -524,6 +524,7 @@ CrExtensionsManagerTestWithMultipleExtensionTypesInstalled =
   /** @override */
   get extraLibraries() {
     return super.extraLibraries.concat([
+      '../settings/test_util.js',
       'manager_test.js',
     ]);
   }

chrome/test/data/webui/extensions/manager_test.js

@@ -34,7 +34,9 @@ cr.define('extension_manager_tests', function() {
     }
 
     setup(function() {
-      manager = document.querySelector('extensions-manager');
+      return test_util.waitForRender(document).then(() => {
+        manager = document.querySelector('extensions-manager');
+      });
     });
 
     test(assert(TestNames.ItemListVisibility), function() {

testing/buildbot/filters/webui_html_imports_polyfill_browser_tests.filter

@@ -3,10 +3,6 @@
 # HTML imports feature turned off, before the Blink team disables the feature by
 # default.
 
-# TODO(rbpotter): Restore this test to the list of tests below, after failures
-# on Mac 10.13 have been fixed.
--CrExtensionsManagerTestWithMultipleExtensionTypesInstalled.*
-
 CrExtensionsSidebarTest.*
 CrExtensionsToolbarTest.*
 CrExtensionsItemsTest.*
@@ -21,6 +17,7 @@ CrExtensionsLoadErrorTest.*
 CrExtensionsManagerUnitTest.*
 CrExtensionsManagerUnitTestWithActivityLogFlag.*
 CrExtensionsManagerTestWithIdQueryParam.*
+CrExtensionsManagerTestWithMultipleExtensionTypesInstalled.*
 CrExtensionsManagerTestWithActivityLogFlag.*
 CrExtensionsShortcutTest.*
 CrExtensionsShortcutInputTest.*