Disable a flaky test PanelTest.LinkMenu in chromevox_tests

Bug: 813320
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Ia7434352408d5327a72d4c9a80da31542ddeba05
Reviewed-on: https://chromium-review.googlesource.com/923770Reviewed-by: David Tseng <dtseng@chromium.org>
Commit-Queue: Takeshi Yoshino <tyoshino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537599}

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

@@ -82,7 +82,8 @@ TEST_F('PanelTest', 'DISABLED_ActivateMenu', function() {
   });
 });
 
-TEST_F('PanelTest', 'LinkMenu', function() {
+// Flaky, see https://crbug.com/813320
+TEST_F('PanelTest', 'DISABLED_LinkMenu', function() {
   var mockFeedback = this.createMockFeedback();
   this.runWithLoadedTree(this.linksDoc, function(root) {
     var openMenus = new PanelCommand(PanelCommandType.OPEN_MENUS, 'role_link');