Migrate off base::ThreadPool()-as-a-trait in /chrome/browser/chromeos/arc/fileapi

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=hashimoto@chromium.org

Bug: 1026641
Change-Id: I6ea3c67e54a44bae1414f39ce1fb343cfb3a2c99
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079073
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745507}

chrome/browser/chromeos/arc/fileapi/file_stream_forwarder.cc

@@ -38,10 +38,10 @@ FileStreamForwarder::FileStreamForwarder(
       remaining_size_(size),
       fd_dest_(std::move(fd_dest)),
       callback_(std::move(callback)),
-      task_runner_(base::CreateSequencedTaskRunner(
+      task_runner_(base::ThreadPool::CreateSequencedTaskRunner(
           // It's safe to shutdown without waiting for the
           // completion of tasks running with this task runner.
-          {base::ThreadPool(), base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN,
+          {base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN,
            base::MayBlock()})),
       buf_(base::MakeRefCounted<net::IOBufferWithSize>(kBufSize)) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);