Migrate off base::ThreadPool()-as-a-trait in /extensions/browser/api/feedback_private

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=jkardatzke@chromium.org

Bug: 1026641
Change-Id: Id008ffcead00c6787d40b56c9370d09b56df87bb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078418
Commit-Queue: J Kardatzke <jkardatzke@chromium.org>
Reviewed-by: J Kardatzke <jkardatzke@chromium.org>
Reviewed-by: Jeffrey Kardatzke <jkardatzke@google.com>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745679}

extensions/browser/api/feedback_private/log_source_access_manager.cc

@@ -10,6 +10,7 @@
 #include "base/bind.h"
 #include "base/strings/string_split.h"
 #include "base/task/post_task.h"
+#include "base/task/thread_pool.h"
 #include "base/task_runner_util.h"
 #include "base/time/default_tick_clock.h"
 #include "extensions/browser/api/api_resource_manager.h"
@@ -74,10 +75,10 @@ void AnonymizeResults(
 LogSourceAccessManager::LogSourceAccessManager(content::BrowserContext* context)
     : context_(context),
       tick_clock_(base::DefaultTickClock::GetInstance()),
-      task_runner_for_anonymizer_(base::CreateSequencedTaskRunner(
+      task_runner_for_anonymizer_(base::ThreadPool::CreateSequencedTaskRunner(
           // User visible as the feedback_api is used by the Chrome (OS)
           // feedback extension while the user may be looking at a spinner.
-          {base::ThreadPool(), base::TaskPriority::USER_VISIBLE,
+          {base::TaskPriority::USER_VISIBLE,
            base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN})),
       anonymizer_container_(
           base::MakeRefCounted<feedback::AnonymizerToolContainer>(