Migrate off base::ThreadPool()-as-a-trait in /chrome/updater

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=ganesh@chromium.org

Bug: 1026641
Change-Id: If0f5732e700bbf9cc8631d53913d3049bec345f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2078424
Commit-Queue: Gabriel Charette <gab@chromium.org>
Auto-Submit: Gabriel Charette <gab@chromium.org>
Reviewed-by: S. Ganesh <ganesh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745480}

chrome/updater/installer.cc

@@ -12,6 +12,7 @@
 #include "base/files/file_util.h"
 #include "base/logging.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 "chrome/updater/action_handler.h"
@@ -27,7 +28,7 @@ namespace {
 
 // This task joins a process, hence .WithBaseSyncPrimitives().
 static constexpr base::TaskTraits kTaskTraitsBlockWithSyncPrimitives = {
-    base::ThreadPool(), base::MayBlock(), base::WithBaseSyncPrimitives(),
+    base::MayBlock(), base::WithBaseSyncPrimitives(),
     base::TaskPriority::BEST_EFFORT,
     base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN};
 
@@ -195,7 +196,7 @@ void Installer::Install(const base::FilePath& unpack_path,
                         const std::string& public_key,
                         std::unique_ptr<InstallParams> install_params,
                         Callback callback) {
-  base::PostTask(
+  base::ThreadPool::PostTask(
       FROM_HERE, kTaskTraitsBlockWithSyncPrimitives,
       base::BindOnce(&Installer::InstallWithSyncPrimitives, this, unpack_path,
                      public_key, std::move(install_params),

chrome/updater/win/net/network_winhttp.cc

@@ -20,7 +20,8 @@
 #include "base/strings/string_piece.h"
 #include "base/strings/stringprintf.h"
 #include "base/strings/sys_string_conversions.h"
-#include "base/task/post_task.h"
+#include "base/task/task_traits.h"
+#include "base/task/thread_pool.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "chrome/updater/win/net/net_util.h"
 #include "chrome/updater/win/net/network.h"
@@ -341,10 +342,10 @@ void NetworkFetcherWinHTTP::RequestError(const WINHTTP_ASYNC_RESULT* result) {
 
 void NetworkFetcherWinHTTP::WriteDataToFile() {
   constexpr base::TaskTraits kTaskTraits = {
-      base::ThreadPool(), base::MayBlock(), base::TaskPriority::BEST_EFFORT,
+      base::MayBlock(), base::TaskPriority::BEST_EFFORT,
       base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN};
 
-  base::PostTaskAndReplyWithResult(
+  base::ThreadPool::PostTaskAndReplyWithResult(
       FROM_HERE, kTaskTraits,
       base::BindOnce(&NetworkFetcherWinHTTP::WriteDataToFileBlocking,
                      base::Unretained(this)),