[dragdrop] Add relatedTarget to drag drop target test events

Setup the event.relatedTarget when creating drag drop test events that
will be sent to the target element.

The browser initializes the event.fromElement from that .relatedTarget
value (use the source element) and is only really needed for dragenter
events to better simulate a dragenter event. Use null for the dragover
and drop events (just like chrome).

Bug: 1062902
Change-Id: I899e704f3026cc1f1cdbce2ed7436fc14be2e74e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2274885
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@{#784245}

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

@@ -842,15 +842,18 @@ test.util.async.fakeDragAndDrop =
             result = source.dispatchEvent(event);
             break;
           case 1:
+            targetOptions.relatedTarget = source;
             event = new DragEvent('dragenter', targetOptions);
             result = target.dispatchEvent(event);
             break;
           case 2:
+            targetOptions.relatedTarget = null;
             event = new DragEvent('dragover', targetOptions);
             result = target.dispatchEvent(event);
             break;
           case 3:
             if (!skipDrop) {
+              targetOptions.relatedTarget = null;
               event = new DragEvent('drop', targetOptions);
               result = target.dispatchEvent(event);
             }