Add note about TabindexOpenDialog/FileManagerBrowserTest

Test disabled. Test is broken when re-enabled. Add TODO note about it,
comment only change.

Bug: 836254
Change-Id: I76d0e8b3bb7f1491a05b55ceb6f3d3adc46f497c
Reviewed-on: https://chromium-review.googlesource.com/1029398Reviewed-by: Stuart Langley <slangley@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553980}

chrome/browser/chromeos/file_manager/file_manager_browsertest.cc

@@ -430,6 +430,7 @@ WRAPPED_INSTANTIATE_TEST_CASE_P(
                                     "tabindexFocusDirectorySelected")));
 
 // Fails on official cros trunk build. http://crbug.com/480491
+// TODO(crbug.com/836254) test broken when DISABLE removed.
 #if defined(DISABLE_SLOW_FILESAPP_TESTS) || defined(OFFICIAL_BUILD)
 #define MAYBE_TabindexOpenDialog DISABLED_TabindexOpenDialog
 #else