Use TaskType::kPostedMessage instead of kInternalMedia at BroardcastChannel

The task type was introduced at https://chromium-review.googlesource.com/c/chromium/src/+/1088534
but kInternalMedia was not appropriate since BroardcastChannel has
nothing to do with media. Use kPostedMessage instead.

Bug: 846618
Change-Id: Icdb72bd88aa4de9b6a7d415dde41268a5d7ca4bc
Reviewed-on: https://chromium-review.googlesource.com/1158314Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Commit-Queue: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579756}

third_party/blink/renderer/modules/broadcastchannel/broadcast_channel.cc

@@ -99,7 +99,7 @@ void BroadcastChannel::OnMessage(BlinkCloneableMessage message) {
   MessageEvent* event = MessageEvent::Create(
       nullptr, std::move(message.message),
       GetExecutionContext()->GetSecurityOrigin()->ToString());
-  EnqueueEvent(event, TaskType::kInternalMedia);
+  EnqueueEvent(event, TaskType::kPostedMessage);
 }
 
 void BroadcastChannel::OnError() {