Fix shutdown hang related to files deletion

This CL is moving the background tasks to "CONTINUE_ON_SHUTDOWN"
shutdown semantic.

If the deletion task has already started before the shutdown started,
it will be executed in low IO priority. This can lead to a really slow
files deletion and can cause shutdown hangs.

Since there is no need to be consistent on these files cleanup, the
shutdown hang can be avoided by letting the task running and not
blocking the shutdown.

R=lucnguyen@google.com, gab@chromium.org

Bug: 1428815
Change-Id: If6c3f40cc12f277de7a7aa49cac624364857c437
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4823246
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Reviewed-by: Luc Nguyen <lucnguyen@google.com>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Reviewed-by: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1189682}

components/metrics/persistent_histograms.cc

@@ -277,10 +277,15 @@ void PersistentHistogramsCleanup(const base::FilePath& metrics_dir) {
       base::Seconds(kSpareFileCreateDelaySeconds));
 
 #if BUILDFLAG(IS_WIN)
+  // Post a best effort task that will delete files. Unlike SKIP_ON_SHUTDOWN,
+  // which will block on the deletion if the task already started,
+  // CONTINUE_ON_SHUTDOWN will not block shutdown on the task completing. It's
+  // not a *necessity* to delete the files the same session they are "detected".
+  // On shutdown, the deletion will be interrupted.
   base::ThreadPool::PostDelayedTask(
       FROM_HERE,
-      {base::MayBlock(), base::TaskPriority::LOWEST,
-       base::TaskShutdownBehavior::SKIP_ON_SHUTDOWN},
+      {base::MayBlock(), base::TaskPriority::BEST_EFFORT,
+       base::TaskShutdownBehavior::CONTINUE_ON_SHUTDOWN},
       base::BindOnce(&DeleteOldWindowsTempFiles, metrics_dir),
       kDeleteOldWindowsTempFilesDelay);
 #endif  // BUILDFLAG(IS_WIN)