Migrate off base::ThreadPool()-as-a-trait in /services/device/usb

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=odejesush@chromium.org

Bug: 1026641
Change-Id: I6fd3ee2edef8a427eb43444f643b750693e58b81
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079081
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Ovidio de Jesús Ruiz-Henríquez <odejesush@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745514}

services/device/usb/usb_service.cc

@@ -8,7 +8,7 @@
 #include "base/feature_list.h"
 #include "base/location.h"
 #include "base/memory/ptr_util.h"
-#include "base/task/post_task.h"
+#include "base/task/thread_pool.h"
 #include "base/threading/sequenced_task_runner_handle.h"
 #include "build/build_config.h"
 #include "components/device_event_log/device_event_log.h"
@@ -64,7 +64,7 @@ std::unique_ptr<UsbService> UsbService::Create() {
 // static
 scoped_refptr<base::SequencedTaskRunner>
 UsbService::CreateBlockingTaskRunner() {
-  return base::CreateSequencedTaskRunner(kBlockingTaskTraits);
+  return base::ThreadPool::CreateSequencedTaskRunner(kBlockingTaskTraits);
 }
 
 UsbService::~UsbService() {

services/device/usb/usb_service.h

@@ -50,10 +50,10 @@ class UsbService {
     virtual void WillDestroyUsbService();
   };
 
-  // These task traits are to be used for posting blocking tasks to the task
-  // scheduler.
+  // These task traits are to be used for posting blocking tasks to the thread
+  // pool.
   static constexpr base::TaskTraits kBlockingTaskTraits = {
-      base::ThreadPool(), base::MayBlock(), base::TaskPriority::USER_VISIBLE,
+      base::MayBlock(), base::TaskPriority::USER_VISIBLE,
       base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN};
 
   // Returns nullptr when initialization fails.

services/device/usb/usb_service_impl.cc

@@ -22,7 +22,7 @@
 #include "base/stl_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/strings/utf_string_conversions.h"
-#include "base/task/post_task.h"
+#include "base/task/thread_pool.h"
 #include "base/threading/scoped_blocking_call.h"
 #include "build/build_config.h"
 #include "components/device_event_log/device_event_log.h"
@@ -232,7 +232,7 @@ void OnDeviceOpenedReadDescriptors(
 UsbServiceImpl::UsbServiceImpl()
     : task_runner_(base::SequencedTaskRunnerHandle::Get()) {
   weak_self_ = weak_factory_.GetWeakPtr();
-  base::PostTaskAndReplyWithResult(
+  base::ThreadPool::PostTaskAndReplyWithResult(
       FROM_HERE, kBlockingTaskTraits,
       base::BindOnce(&InitializeUsbContextBlocking),
       base::BindOnce(&UsbServiceImpl::OnUsbContext,
@@ -332,7 +332,7 @@ void UsbServiceImpl::RefreshDevices() {
     pending_path_enumerations_.pop();
   }
 
-  base::PostTaskAndReplyWithResult(
+  base::ThreadPool::PostTaskAndReplyWithResult(
       FROM_HERE, kBlockingTaskTraits,
       base::BindOnce(&GetDeviceListBlocking, device_path, context_),
       base::BindOnce(&UsbServiceImpl::OnDeviceList,