Migrate off base::ThreadPool()-as-a-trait in /chrome/browser/ui/webui/policy_ui_handler.cc

This CL is a logical no-op.

It manually covers the use cases the script
(https://crbug.com/1026641#c22) didn't cover in
https://chromium-review.googlesource.com/c/chromium/src/+/2026350.

A few of the affected files were also migrated to
content::GetUIThreadTaskRunner() whenever surrounding code consistency
was at stake (but the bulk of that migration is staged to happen after
this one so this wasn't a priority in this batch).

Most files intentionally kept base/task/post_task.h as an include. It is
still required because of a cyclical dependency during the migration
preventing base/task/thread_pool.h from implicitly providing
base/task/task_traits.h to its users (callers that explicitly store
TaskTraits and include task_traits.h don't have this problem). The
post_task.h include will be cleaned up in a later phase.

This CL was uploaded by git cl split.

R=pastarmovj@chromium.org

Bug: 1026641
Change-Id: I8d1d6f3c2d6da8d599078e727b0c27587f664017
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078661
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Julian Pastarmov <pastarmovj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745506}

chrome/browser/ui/webui/policy_ui_handler.cc

@@ -818,10 +818,9 @@ void PolicyUIHandler::RegisterMessages() {
 
 #if defined(OS_WIN) && BUILDFLAG(GOOGLE_CHROME_BRANDING)
   base::PostTaskAndReplyWithResult(
-      base::CreateCOMSTATaskRunner(
+      base::ThreadPool::CreateCOMSTATaskRunner(
           {base::TaskPriority::USER_BLOCKING,
-           base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN, base::MayBlock(),
-           base::ThreadPool()})
+           base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN, base::MayBlock()})
           .get(),
       FROM_HERE, base::BindOnce(&GetGoogleUpdatePolicies),
       base::BindOnce(&PolicyUIHandler::SetUpdaterPolicies,