Change ack timeout delayed task priority to USER_VISIBLE

This would avoid the timer not firing in background for Android

Bug: 1001098
Change-Id: I18b8bb3a6b9fdc55a159bd936554c9331392cf83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1789584
Commit-Queue: Alex Chau <alexchau@chromium.org>
Commit-Queue: Richard Knoll <knollr@chromium.org>
Auto-Submit: Alex Chau <alexchau@chromium.org>
Reviewed-by: Richard Knoll <knollr@chromium.org>
Reviewed-by: Michael van Ouwerkerk <mvanouwerkerk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#694238}

chrome/browser/sharing/sharing_service.cc

@@ -192,7 +192,7 @@ void SharingService::SendMessageToDevice(
   send_message_callbacks_.emplace(message_guid, std::move(callback));
 
   base::PostDelayedTask(
-      FROM_HERE, {base::TaskPriority::BEST_EFFORT, content::BrowserThread::UI},
+      FROM_HERE, {base::TaskPriority::USER_VISIBLE, content::BrowserThread::UI},
       base::BindOnce(&SharingService::InvokeSendMessageCallback,
                      weak_ptr_factory_.GetWeakPtr(), message_guid,
                      SharingSendMessageResult::kAckTimeout),