Remove some Event::SetTarget for MediaElementEventQueue

Before merging MediaElementEventQueue into EventQueueImpl, we need to
eliminate code to set the target for an event. This is part of this work.

Design Doc: https://docs.google.com/document/d/1BBtBPTarOF4NeVKSWZe3XaDHo4yTGhdlqYm35yVzPs4/edit#

Bug: 846618
Change-Id: Ic3b19ff2af19d931356ff72800797b36506f5ff0
Reviewed-on: https://chromium-review.googlesource.com/1102233
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567587}

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

@@ -90,6 +90,7 @@ void MediaElementEventQueue::DispatchEvent(Event* event) {
   probe::AsyncTask async_task(GetExecutionContext(), event);
   TRACE_EVENT_ASYNC_STEP_INTO1("event", "MediaElementEventQueue:enqueueEvent",
                                event, "dispatch", "type", type);
+  // TODO(hajimehoshi): Always use |owner_| instead of |event->target()|
   EventTarget* target = event->target() ? event->target() : owner_.Get();
   target->DispatchEvent(event);
   TRACE_EVENT_ASYNC_END1("event", "MediaElementEventQueue:enqueueEvent", event,

third_party/blink/renderer/modules/encryptedmedia/media_key_session.cc

@@ -876,7 +876,6 @@ void MediaKeySession::Message(MessageType message_type,
 
   MediaKeyMessageEvent* event =
       MediaKeyMessageEvent::Create(EventTypeNames::message, init);
-  event->SetTarget(this);
   async_event_queue_->EnqueueEvent(FROM_HERE, event);
 }
 
@@ -961,7 +960,6 @@ void MediaKeySession::KeysStatusesChange(
   // 5. Queue a task to fire a simple event named keystatuseschange
   //    at the session.
   Event* event = Event::Create(EventTypeNames::keystatuseschange);
-  event->SetTarget(this);
   async_event_queue_->EnqueueEvent(FROM_HERE, event);
 
   // 6. Queue a task to run the attempt to resume playback if necessary

third_party/blink/renderer/modules/mediasource/media_source.cc

@@ -846,8 +846,6 @@ void MediaSource::ScheduleEvent(const AtomicString& event_name) {
   DCHECK(async_event_queue_);
 
   Event* event = Event::Create(event_name);
-  event->SetTarget(this);
-
   async_event_queue_->EnqueueEvent(FROM_HERE, event);
 }