Post auth transfer task on same sequence

Before this CL, it was posted on the ThreadPool, however the task assumes that
it runs on the same sequence it was posted from.

This gave me DHCECK failures in browsertests, but could have worse
implications.

Bug: none
Test: browser_tests don't fail with with DCHECK failures for me anymore
Change-Id: I583bbd5e483568fa39595e6e7c4c854570998758
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2055085
Commit-Queue: Pavol Marko <pmarko@chromium.org>
Reviewed-by: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741535}

chrome/browser/chromeos/login/existing_user_controller.cc

@@ -23,6 +23,7 @@
 #include "base/strings/string_util.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/task/post_task.h"
+#include "base/threading/sequenced_task_runner_handle.h"
 #include "base/values.h"
 #include "base/version.h"
 #include "chrome/app/vector_icons/vector_icons.h"
@@ -573,7 +574,7 @@ void ExistingUserController::Observe(
   // has been updated before we copy it.
   // TODO(pmarko): Find a better way to do this, see https://crbug.com/796512.
   VLOG(1) << "Authentication was entered manually, possibly for proxyauth.";
-  base::PostDelayedTask(
+  base::SequencedTaskRunnerHandle::Get()->PostDelayedTask(
       FROM_HERE, base::BindOnce(&TransferHttpAuthCaches),
       base::TimeDelta::FromMilliseconds(kAuthCacheTransferDelayMs));
 }