Revert "Remove some Event::SetTarget for MediaElementEventQueue"

This reverts commit 60d31377.

Reason for revert: We realized that MediaElementEventQueue : target is not 1:1. Instead of MediaElementEventQueue with a target, each event should have its target.

Original change's description:
> 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}

TBR=hajimehoshi@chromium.org,haraken@chromium.org,altimin@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 846618
Change-Id: I28cf7c58e123b66e2472f64b233684cbf1006dd4
Reviewed-on: https://chromium-review.googlesource.com/1105540Reviewed-by: Hajime Hoshi <hajimehoshi@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568370}

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

@@ -90,7 +90,6 @@ 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

@@ -876,6 +876,7 @@ void MediaKeySession::Message(MessageType message_type,
 
   MediaKeyMessageEvent* event =
       MediaKeyMessageEvent::Create(EventTypeNames::message, init);
+  event->SetTarget(this);
   async_event_queue_->EnqueueEvent(FROM_HERE, event);
 }
 
@@ -960,6 +961,7 @@ 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

@@ -846,6 +846,8 @@ 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);
 }