Move on-demand update checks from the FILE thread to the blocking pool.

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

Index: chrome/browser/ui/webui/help/version_updater_win.cc
diff --git a/chrome/browser/ui/webui/help/version_updater_win.cc b/chrome/browser/ui/webui/help/version_updater_win.cc
index e0eb626dc437512ab61e19ceeaa435f2aabc6e8c..2613161e6f492e620f004f6243ffbf00ce296463 100644
--- a/chrome/browser/ui/webui/help/version_updater_win.cc
+++ b/chrome/browser/ui/webui/help/version_updater_win.cc
@@ -39,7 +39,7 @@ void VersionUpdaterWin::CheckForUpdate(const StatusCallback& callback,
         (base::win::OSInfo::GetInstance()->service_pack().major == 0) &&
         !base::win::UserAccountControlIsEnabled())) {
     callback_.Run(CHECKING, 0, base::string16());
-    BeginUpdateCheckOnFileThread(false /* !install_update_if_possible */);
+    BeginUpdateCheckInBlockingPool(false /* !install_update_if_possible */);
   }
 }
 
@@ -66,7 +66,7 @@ void VersionUpdaterWin::OnUpdateCheckComplete(
   } else {
     // Notify the caller that the update is now beginning and initiate it.
     status = UPDATING;
-    BeginUpdateCheckOnFileThread(true /* install_update_if_possible */);
+    BeginUpdateCheckInBlockingPool(true /* install_update_if_possible */);
   }
   callback_.Run(status, 0, base::string16());
 }
@@ -113,10 +113,13 @@ void VersionUpdaterWin::OnError(GoogleUpdateErrorCode error_code,
   callback_.Run(status, 0, message);
 }
 
-void VersionUpdaterWin::BeginUpdateCheckOnFileThread(
+void VersionUpdaterWin::BeginUpdateCheckInBlockingPool(
     bool install_update_if_possible) {
-  BeginUpdateCheck(content::BrowserThread::GetTaskRunnerForThread(
-                       content::BrowserThread::FILE),
+  base::SequencedWorkerPool* blocking_pool =
+      content::BrowserThread::GetBlockingPool();
+  BeginUpdateCheck(blocking_pool->GetSequencedTaskRunnerWithShutdownBehavior(
+                       blocking_pool->GetSequenceToken(),
+                       base::SequencedWorkerPool::CONTINUE_ON_SHUTDOWN),

[Peter Kasting, 2016/08/03 01:24:34]

The comments on CONTINUE_ON_SHUTDOWN warn that you could get called back
anytime, even in the midst of final app teardown, so anything global could be
garbage.  Make sure this doesn't cause you any problems...

[grt (UTC plus 2), 2016/08/03 05:36:31]

I did consider this, and believe it to be safe. Nothing done in the
UpdateCheckDriver touches any global state as far as I know (e.g.,
g_browser_process). Notifications back through the UpdateCheckDelegate all take
place on the caller's SequencedTaskRunner. PostTask to that thread from the
blocking pool will return false if it's gone, and will work normally if not
(it's no big deal if those tasks are dropped on the floor). Let me know if
you're aware of other considerations. Thanks.

                    g_browser_process->GetApplicationLocale(),
                    install_update_if_possible, owner_widget_,
                    weak_factory_.GetWeakPtr());