Add DCHECK to check event target existence at EventQueue

This check was in MediaElementEventQueue but lost when merging this into
EventQueueImpl.

Bug: 846618
Change-Id: I354ab7a4787624c6269e6abd0a67eb0a63680ad6
Reviewed-on: https://chromium-review.googlesource.com/1113397Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#570016}

third_party/blink/renderer/core/dom/events/event_queue.cc

@@ -57,6 +57,7 @@ bool EventQueue::EnqueueEvent(const base::Location& from_here, Event* event) {
   if (is_closed_)
     return false;
 
+  DCHECK(event->target());
   DCHECK(GetExecutionContext());
 
   probe::AsyncTaskScheduled(GetExecutionContext(), event->type(), event);