Show minimized thumbnail on download notification.

When an image is downloaded, download notification should show minimized
thumbnail on the right when collapsed, just like screenshot notification.

For the explanation with a screenshot, see the bug.

TEST=manual
BUG=766900

Change-Id: Id9ab63cb5c77fede9a9fb9502232c300c22a94a9
Reviewed-on: https://chromium-review.googlesource.com/674464Reviewed-by: Min Qin <qinmin@chromium.org>
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Cr-Commit-Position: refs/heads/master@{#503639}

chrome/browser/download/notification/download_item_notification.cc

@@ -420,6 +420,7 @@ void DownloadItemNotification::UpdateNotificationData(
   if (message_center::IsNewStyleNotificationEnabled()) {
     notification_->set_message(GetSubStatusString());
     notification_->set_progress_status(GetStatusString());
+    notification_->set_use_image_as_icon(true);
   } else {
     notification_->set_message(GetStatusString());
   }