Migrate off base::ThreadPool()-as-a-trait in /components/upload_list

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=mark@chromium.org

Bug: 1026641
Change-Id: I38da1f6c3416d9f3539129fcb0977d3edc5980e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079151
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745489}

components/upload_list/upload_list.cc

@@ -9,15 +9,15 @@
 #include <utility>
 
 #include "base/bind.h"
-#include "base/task/post_task.h"
 #include "base/task/task_traits.h"
+#include "base/task/thread_pool.h"
 
 namespace {
 
 // USER_VISIBLE because loading uploads blocks chrome://crashes,
 // chrome://webrtc-logs and the feedback UI. See https://crbug.com/972526.
 constexpr base::TaskTraits kLoadingTaskTraits = {
-    base::ThreadPool(), base::MayBlock(), base::TaskPriority::USER_BLOCKING,
+    base::MayBlock(), base::TaskPriority::USER_BLOCKING,
     base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN};
 
 }  // namespace
@@ -63,7 +63,7 @@ UploadList::~UploadList() = default;
 void UploadList::Load(base::OnceClosure callback) {
   DCHECK(sequence_checker_.CalledOnValidSequence());
   load_callback_ = std::move(callback);
-  base::PostTaskAndReplyWithResult(
+  base::ThreadPool::PostTaskAndReplyWithResult(
       FROM_HERE, kLoadingTaskTraits,
       base::BindOnce(&UploadList::LoadUploadList, this),
       base::BindOnce(&UploadList::OnLoadComplete, this));
@@ -74,7 +74,7 @@ void UploadList::Clear(const base::Time& begin,
                        base::OnceClosure callback) {
   DCHECK(sequence_checker_.CalledOnValidSequence());
   clear_callback_ = std::move(callback);
-  base::PostTaskAndReply(
+  base::ThreadPool::PostTaskAndReply(
       FROM_HERE, kLoadingTaskTraits,
       base::BindOnce(&UploadList::ClearUploadList, this, begin, end),
       base::BindOnce(&UploadList::OnClearComplete, this));
@@ -86,7 +86,7 @@ void UploadList::CancelLoadCallback() {
 
 void UploadList::RequestSingleUploadAsync(const std::string& local_id) {
   DCHECK(sequence_checker_.CalledOnValidSequence());
-  base::PostTask(
+  base::ThreadPool::PostTask(
       FROM_HERE, kLoadingTaskTraits,
       base::BindOnce(&UploadList::RequestSingleUpload, this, local_id));
 }