cros: Disable flaky TrayAccessibilityTest.CheckMenuVisibilityOnDetailMenu

Flakily fails with:
../../chrome/browser/chromeos/system/tray_accessibility_browsertest.cc:1681: Failure
Value of: IsHelpAvailableOnDetailMenu()
  Actual: false
Expected: true

NOTRY=true
TBR=xiyuan@chromium.org

Bug: 819987, 787024
Change-Id: I133007c1411f714f2b7646c0c0d949c577c63037
Reviewed-on: https://chromium-review.googlesource.com/956276
Commit-Queue: James Cook <jamescook@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541980}

chrome/browser/chromeos/system/tray_accessibility_browsertest.cc

@@ -1668,7 +1668,10 @@ IN_PROC_BROWSER_TEST_P(TrayAccessibilityTest, CheckMarksOnDetailMenu) {
   CloseDetailMenu();
 }
 
-IN_PROC_BROWSER_TEST_P(TrayAccessibilityTest, CheckMenuVisibilityOnDetailMenu) {
+// Flaky failures of IsHelpAvailableOnDetailMenu(). Possible race condition
+// with session state. https://crbug.com/819987
+IN_PROC_BROWSER_TEST_P(TrayAccessibilityTest,
+                       DISABLED_CheckMenuVisibilityOnDetailMenu) {
   // Except help & settings, others should be kept the same
   // in LOGIN | NOT LOGIN | LOCKED. https://crbug.com/632107.
   EXPECT_TRUE(CreateDetailedMenu());