Add note about TabindexSaveFileDialog/FileManagerBrowserTest

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

Tbr: lucmult
No-Try: true
Bug: 836254
Change-Id: If02734d34dbcbd64c7ac294bbeda995bc564d9bb
Reviewed-on: https://chromium-review.googlesource.com/1030110Reviewed-by: Noel Gordon <noel@chromium.org>
Commit-Queue: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553983}

chrome/browser/chromeos/file_manager/file_manager_browsertest.cc

@@ -445,6 +445,7 @@ WRAPPED_INSTANTIATE_TEST_CASE_P(
         TestParameter(IN_GUEST_MODE, "tabindexOpenDialogDownloads")));
 
 // Fails on official build. http://crbug.com/482121.
+// TODO(crbug.com/836254) test broken when DISABLE removed.
 #if defined(DISABLE_SLOW_FILESAPP_TESTS) || defined(OFFICIAL_BUILD)
 #define MAYBE_TabindexSaveFileDialog DISABLED_TabindexSaveFileDialog
 #else