Extensions Web UI: Fix test to pass on Mac with HTML imports disabled

Fix CrExtensionsManagerTestWithMultipleExtensionTypesInstalled.* to
pass on Mac 10.13 with --disable-blink-features=HTMLImports

Reproduced test failure locally on Mac 10.13, and verified test passes
with fix.

Bug: 925517
Change-Id: I8cabf2fccc3c4a1660373d65d203854e1234664c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1604204Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658728}

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',
     ]);
   }
@@ -570,6 +571,7 @@ CrExtensionsManagerTestWithIdQueryParam =
   /** @override */
   get extraLibraries() {
     return super.extraLibraries.concat([
+      '../settings/test_util.js',
       'manager_test.js',
     ]);
   }

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

@@ -35,6 +35,12 @@ cr.define('extension_manager_tests', function() {
 
     setup(function() {
       manager = document.querySelector('extensions-manager');
+      // Wait for the first view to be active before starting tests, if one is
+      // not active already. Sometimes, on Mac with native HTML imports
+      // disabled, no views are active at this point.
+      return manager.$.viewManager.querySelector('.active') ?
+          Promise.resolve() :
+          test_util.eventToPromise('view-enter-start', 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.*
@@ -20,6 +16,7 @@ CrExtensionsItemListTest.*
 CrExtensionsLoadErrorTest.*
 CrExtensionsManagerUnitTest.*
 CrExtensionsManagerUnitTestWithActivityLogFlag.*
+CrExtensionsManagerTestWithMultipleExtensionTypesInstalled.*
 CrExtensionsManagerTestWithIdQueryParam.*
 CrExtensionsManagerTestWithActivityLogFlag.*
 CrExtensionsShortcutTest.*