Reenable ChromeVoxBackgroundTest.ListItem

This test once timed out if LayoutNG and VizDisplayCompositor
were enabled.

This seems to pass on the current bot. This patch tries to
reenable.

Bug: 959261
Change-Id: I818fcc015f9e6254fa7f8c17588d6406efeabe67
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2354375Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Koji Ishii <kojii@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801464}

chrome/browser/resources/chromeos/accessibility/chromevox/background/background_test.js

@@ -1263,9 +1263,7 @@ TEST_F('ChromeVoxBackgroundTest', 'InPageLinks', function() {
       });
 });
 
-// Timeout on ChromeOS with LayoutNG & Viz.
-// https://crbug.com/959261
-TEST_F('ChromeVoxBackgroundTest', 'DISABLED_ListItem', function() {
+TEST_F('ChromeVoxBackgroundTest', 'ListItem', function() {
   this.resetContextualOutput();
   const mockFeedback = this.createMockFeedback();
   this.runWithLoadedTree(