Disable flaky ChromeVoxLiveRegionsUnitTest.AddToLiveRegion

TBR=dmazzoni@chromium.org
NOTRY=true

Change-Id: Ib20e44b272d84612d480bf3cccf4bbf8dec20fa1
Bug: 498881
Reviewed-on: https://chromium-review.googlesource.com/c/1355323
Commit-Queue: danakj <danakj@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612230}

chrome/browser/resources/chromeos/chromevox/chromevox/injected/live_regions_test.unitjs

@@ -131,7 +131,8 @@ TEST_F('ChromeVoxLiveRegionsUnitTest', 'ModifyStatusLiveRegion', function() {
 /**
  * Test adding element to a atomic and non-atomic live regions.
  */
-TEST_F('ChromeVoxLiveRegionsUnitTest', 'AddToLiveRegion', function() {
+// Flaky on Chromium OS: crbug.com/909973.
+TEST_F('ChromeVoxLiveRegionsUnitTest', 'DISABLED_AddToLiveRegion', function() {
   this.loadDoc(function() {/*!
     <div>
       <div id="non_atomic_buddylist" aria-live="polite">