Fallback to legacy type only when event is trusted

According to the spec[1], |isTrusted| should be tested before
fallback to the legacy event type.

4 failed tests will be passed after this change.

[1] https://dom.spec.whatwg.org/#concept-event-listener-invoke

Bug: 692000
Change-Id: I6f890f738d96b179e404f6db90f7ebe5041c351c
Reviewed-on: https://chromium-review.googlesource.com/547448
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#482482}

third_party/WebKit/Source/core/events/EventTarget.cpp

@@ -621,7 +621,7 @@ DispatchEventResult EventTarget::FireEventListeners(Event* event) {
   bool fired_event_listeners = false;
   if (listeners_vector) {
     fired_event_listeners = FireEventListeners(event, d, *listeners_vector);
-  } else if (legacy_listeners_vector) {
+  } else if (event->isTrusted() && legacy_listeners_vector) {
     AtomicString unprefixed_type_name = event->type();
     event->SetType(legacy_type_name);
     fired_event_listeners =