Change task priority to load icon from file.

When loading icon from file thread, user may say default package icon on
shelf/launcher. Change the task priority to USER_VISIBLE.
The change would also match the description of TaskPriority::USER_VISIBLE.
"Loading an image that is displayed in the UI but is non-critical"

Bug: 1010311
Bug: 997875
Change-Id: Ic6a96b1fc1aef533f1cc9e9192a860d3cbe5a34f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1842295Reviewed-by: Yury Khmel <khmel@chromium.org>
Commit-Queue: Long Cheng <lgcheng@google.com>
Cr-Commit-Position: refs/heads/master@{#703022}

chrome/browser/ui/app_list/arc/arc_app_icon.cc

@@ -305,7 +305,7 @@ void ArcAppIcon::LoadForScaleFactor(ui::ScaleFactor scale_factor) {
 
   base::PostTaskAndReplyWithResult(
       FROM_HERE,
-      {base::ThreadPool(), base::MayBlock(), base::TaskPriority::BEST_EFFORT},
+      {base::ThreadPool(), base::MayBlock(), base::TaskPriority::USER_VISIBLE},
       base::BindOnce(
           &ArcAppIcon::ReadOnFileThread, scale_factor, path,
           prefs->MaybeGetIconPathForDefaultApp(mapped_app_id_, descriptor)),

chrome/browser/ui/app_list/crostini/crostini_app_icon.cc

@@ -217,7 +217,7 @@ void CrostiniAppIcon::LoadForScaleFactor(ui::ScaleFactor scale_factor) {
 
   base::PostTaskAndReplyWithResult(
       FROM_HERE,
-      {base::ThreadPool(), base::MayBlock(), base::TaskPriority::BEST_EFFORT},
+      {base::ThreadPool(), base::MayBlock(), base::TaskPriority::USER_VISIBLE},
       base::BindOnce(&CrostiniAppIcon::ReadOnFileThread, scale_factor, path),
       base::BindOnce(&CrostiniAppIcon::OnIconRead,
                      weak_ptr_factory_.GetWeakPtr()));