[OsSettingsSearch] Fix flaky browsertest.

Wait for iron list to complete resizing, then flush to ensure
all os-search-result-rows are loaded.

(https://logs.chromium.org/logs/chrome/buildbucket/cr-buildbucket.appspot.com/8883704380960662928/+/steps/browser_tests/0/logs/Deterministic_failure:_OSSettingsSearchBoxBrowserTest.AllJsTests__status_FAILURE_/0)

Fixed: 1068811
Change-Id: I15b35de45399c13cd10606980cf5c00dcbf0562a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2140203Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758271}

chrome/test/data/webui/settings/chromeos/os_settings_browsertest.js

@@ -755,7 +755,7 @@ var OSSettingsSearchBoxBrowserTest = class extends OSSettingsBrowserTest {
   }
 };
 
-TEST_F('OSSettingsSearchBoxBrowserTest', 'DISABLED_AllJsTests', () => {
+TEST_F('OSSettingsSearchBoxBrowserTest', 'AllJsTests', () => {
   mocha.run();
 });
 

chrome/test/data/webui/settings/chromeos/os_settings_search_box_test.js

@@ -35,6 +35,12 @@ suite('OSSettingsSearchBox', () => {
     Polymer.dom.flush();
   }
 
+  async function waitForListUpdate() {
+    // Wait for iron-list to complete resizing.
+    await test_util.eventToPromise('iron-resize', resultList);
+    Polymer.dom.flush();
+  }
+
   /**
    * @param {string} resultText Exact string of the result to be displayed.
    * @param {string} path Url path with optional params.
@@ -187,12 +193,7 @@ suite('OSSettingsSearchBox', () => {
     settingsSearchHandler.setFakeResults(
         [fakeResult('WiFi Settings', 'networks?type=WiFi')]);
     await simulateSearch('query');
-
-    // Adding two flushTasks ensures that all events are fully handled, so that
-    // the iron-list is fully rendered and that the physical
-    // <os-search-result-row>s are available to dispatch 'Enter' on.
-    await test_util.flushTasks();
-    await test_util.flushTasks();
+    await waitForListUpdate();
 
     const enterEvent = new KeyboardEvent(
         'keydown', {cancelable: true, key: 'Enter', keyCode: 13});
@@ -210,12 +211,7 @@ suite('OSSettingsSearchBox', () => {
     settingsSearchHandler.setFakeResults(
         [fakeResult('WiFi Settings', 'networks?type=WiFi')]);
     await simulateSearch('query');
-
-    // Adding two flushTasks ensures that all events are fully handled, so that
-    // the iron-list is fully rendered and that the physical
-    // <os-search-result-row>s are available to dispatch 'Enter' on.
-    await test_util.flushTasks();
-    await test_util.flushTasks();
+    await waitForListUpdate();
 
     const searchResultRow = searchBox.getSelectedOsSearchResultRow_();