Bug 1543350 - don't shut-down taskqueue early. r=jya,mjf

Shutting down the taskqueue early prevents the decoder's tasks to be queued.

A TaskQueue no longer requires to be explicitly shutdown it will shutdown when ref counter drops to zero.

Differential Revision: https://phabricator.services.mozilla.com/D26929

--HG--
extra : moz-landing-system : lando

dom/media/ipc/RemoteDecoderParent.cpp

@@ -159,9 +159,6 @@ void RemoteDecoderParent::ActorDestroy(ActorDestroyReason aWhy) {
     mDecoder->Shutdown();
     mDecoder = nullptr;
   }
-  if (mDecodeTaskQueue) {
-    mDecodeTaskQueue->BeginShutdown();
-  }
 }
 
 void RemoteDecoderParent::Error(const MediaResult& aError) {