Disable ChromeVoxBackgroundTest.EventFromUser

It has become very flaky in the past couple of days.

Bug: 1018336
Change-Id: Ieff3a1dfb16fdffde196d1192e752c912499c73a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1880543
Commit-Queue: Aaron Krajeski <aaronhk@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709651}

chrome/browser/resources/chromeos/chromevox/cvox2/background/background_test.extjs

@@ -1961,7 +1961,7 @@ TEST_F('ChromeVoxBackgroundTest', 'EventFromAction', function() {
   });
 });
 
-TEST_F('ChromeVoxBackgroundTest', 'EventFromUser', function() {
+TEST_F('ChromeVoxBackgroundTest', 'DISABLED_EventFromUser', function() {
   this.runWithLoadedTree(function() {/*
     <button>ok</button><button>cancel</button>
   */}, function(root) {