Notifications: Remove system priority for screenshot notification.

With system priority, the screenshot notification never auto dismisses and
remains forever.

BUG=987928

Change-Id: Ib83a156444970233b6d95654749dbd1cc9c4013e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1726764Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Tim Song <tengs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682810}

chrome/browser/ui/ash/chrome_screenshot_grabber.cc

@@ -685,7 +685,6 @@ void ChromeScreenshotGrabber::OnReadScreenshotFileForPreviewCompleted(
                                                     screenshot_path),
           kNotificationImageIcon,
           message_center::SystemNotificationWarningLevel::NORMAL);
-  notification->SetSystemPriority();
 
   NotificationDisplayService::GetForProfile(GetProfile())
       ->Display(NotificationHandler::Type::TRANSIENT, *notification,

chrome/browser/ui/ash/chrome_screenshot_grabber_browsertest.cc

@@ -105,7 +105,6 @@ IN_PROC_BROWSER_TEST_F(ChromeScreenshotGrabberBrowserTest, TakeScreenshot) {
   auto notification =
       display_service_->GetNotification(std::string("screenshot"));
   ASSERT_TRUE(notification.has_value());
-  EXPECT_EQ(message_center::SYSTEM_PRIORITY, notification->priority());
   EXPECT_EQ(message_center::NotifierType::SYSTEM_COMPONENT,
             notification->notifier_id().type);
   EXPECT_EQ("ash.screenshot", notification->notifier_id().id);