[dragdrop] Tests should send a 'dragend' event

The test drag and drop test helper sends a 'dragEnd' event but nothing
responds to that event in files app, nor is it defined in the spec.

Make it spec-conformant: change 'dragEnd' => 'dragend' and the browser
will properly handle it and files app will correctly process it.

Bug: 1062902
Change-Id: I3ed0ba7c7e5f586132dcb88f34e5537ff7587a08
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2276163
Commit-Queue: Alex Danilo <adanilo@chromium.org>
Auto-Submit: Noel Gordon <noel@chromium.org>
Reviewed-by: Alex Danilo <adanilo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#784244}

ui/file_manager/file_manager/background/js/runtime_loaded_test_util.js

@@ -857,7 +857,7 @@ test.util.async.fakeDragAndDrop =
             break;
           case 4:
             if (!skipDrop) {
-              event = new DragEvent('dragEnd', sourceOptions);
+              event = new DragEvent('dragend', sourceOptions);
               result = source.dispatchEvent(event);
             }
             break;