Revert "Replace GetBlockingPool and FILE thread with post_task API"

This reverts commit d9339152.

Reason for revert: <INSERT REASONING HERE>
Causing compile error here:
https://build.chromium.org/p/chromium.chrome/builders/Google%20Chrome%20Win/builds/19349

Original change's description:
> Replace GetBlockingPool and FILE thread with post_task API
> 
> BUG=689520,667892
> 
> Change-Id: I1143fbc8c5f7c12dfdd8e5764d58c9b372d3cb16
> Reviewed-on: https://chromium-review.googlesource.com/544782
> Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
> Reviewed-by: Greg Thompson <grt@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#481889}

TBR=xiyuan@chromium.org,grt@chromium.org

Change-Id: I4fbdb87edffff078ca60bb16c10628b5b13da00a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 689520, 667892
Reviewed-on: https://chromium-review.googlesource.com/546115Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#481902}

chrome/browser/ui/webui/help/version_updater_win.cc

@@ -6,7 +6,7 @@
 
 #include "base/memory/weak_ptr.h"
 #include "base/task_runner_util.h"
-#include "base/task_scheduler/post_task.h"
+#include "base/threading/sequenced_worker_pool.h"
 #include "base/win/win_util.h"
 #include "base/win/windows_version.h"
 #include "chrome/browser/browser_process.h"
@@ -40,7 +40,7 @@ void VersionUpdaterWin::CheckForUpdate(const StatusCallback& callback,
         (base::win::OSInfo::GetInstance()->service_pack().major == 0) &&
         !base::win::UserAccountControlIsEnabled())) {
     callback_.Run(CHECKING, 0, std::string(), 0, base::string16());
-    BeginUpdateCheckInBackground(false /* !install_update_if_possible */);
+    BeginUpdateCheckOnFileThread(false /* !install_update_if_possible */);
   }
 }
 
@@ -51,13 +51,12 @@ void VersionUpdaterWin::OnUpdateCheckComplete(
   if (new_version.empty()) {
     // Google Update says that no new version is available. Check to see if a
     // restart is needed for a previously-applied update to take effect.
-    if (base::PostTaskWithTraitsAndReplyWithResult(
+    if (base::PostTaskAndReplyWithResult(
+            content::BrowserThread::GetBlockingPool(),
             FROM_HERE,
-            {base::TaskPriority::USER_VISIBLE, base::MayBlock(),
-             base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN},
-            base::BindOnce(&upgrade_util::IsUpdatePendingRestart),
-            base::BindOnce(&VersionUpdaterWin::OnPendingRestartCheck,
-                           weak_factory_.GetWeakPtr()))) {
+            base::Bind(&upgrade_util::IsUpdatePendingRestart),
+            base::Bind(&VersionUpdaterWin::OnPendingRestartCheck,
+                       weak_factory_.GetWeakPtr()))) {
       // Early exit since callback_ will be Run in OnPendingRestartCheck.
       return;
     }
@@ -68,7 +67,7 @@ void VersionUpdaterWin::OnUpdateCheckComplete(
   } else {
     // Notify the caller that the update is now beginning and initiate it.
     status = UPDATING;
-    BeginUpdateCheckInBackground(true /* install_update_if_possible */);
+    BeginUpdateCheckOnFileThread(true /* install_update_if_possible */);
   }
   callback_.Run(status, 0, std::string(), 0, base::string16());
 }
@@ -115,13 +114,12 @@ void VersionUpdaterWin::OnError(GoogleUpdateErrorCode error_code,
   callback_.Run(status, 0, std::string(), 0, message);
 }
 
-void VersionUpdaterWin::BeginUpdateCheckInBackground(
+void VersionUpdaterWin::BeginUpdateCheckOnFileThread(
     bool install_update_if_possible) {
   // Disconnect from any previous attempts to avoid redundant callbacks.
   weak_factory_.InvalidateWeakPtrs();
-  BeginUpdateCheck(base::CreateCOMSTATaskRunnerWithTraits(
-                       (base::TaskPriority::USER_VISIBLE, base::MayBlock(),
-                        base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN)),
+  BeginUpdateCheck(content::BrowserThread::GetTaskRunnerForThread(
+                       content::BrowserThread::FILE),
                    g_browser_process->GetApplicationLocale(),
                    install_update_if_possible, owner_widget_,
                    weak_factory_.GetWeakPtr());

chrome/browser/ui/webui/help/version_updater_win.h

@@ -38,7 +38,7 @@ class VersionUpdaterWin : public VersionUpdater,
                const base::string16& new_version) override;
 
  private:
-  void BeginUpdateCheckInBackground(bool install_update_if_possible);
+  void BeginUpdateCheckOnFileThread(bool install_update_if_possible);
 
   // A task run on the UI thread with the result of checking for a pending
   // restart.