Add CHECKs to track down crbug/882574

Bug: 882574
Change-Id: I7a5e3bba87410f4e5c41804d102e88e8426116d8
Reviewed-on: https://chromium-review.googlesource.com/1238194
Commit-Queue: Fergal Daly <fergal@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593151}

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

@@ -187,6 +187,9 @@ DispatchEventResult EventDispatcher::Dispatch() {
                               pre_dispatch_event_handler_result) ==
       kContinueDispatching) {
     if (DispatchEventAtCapturing() == kContinueDispatching) {
+      // TODO(crbug/882574): Remove these.
+      CHECK(event_->HasEventPath());
+      CHECK(!event_->GetEventPath().IsEmpty());
       if (DispatchEventAtTarget() == kContinueDispatching)
         DispatchEventAtBubbling();
     }