Remove unnecessary posted task in cast_transport.cc.

The original author can't remember why this post was necessary. Since
the actual result callback is posted immediately afterwards any
side-effects (such as synchronizing with another message loop) should
already be achieved.

Review-Url: https://codereview.chromium.org/2044603002
Cr-Commit-Position: refs/heads/master@{#398124}

extensions/browser/api/cast_channel/cast_transport.cc

@@ -327,8 +327,6 @@ int CastTransportImpl::DoWriteCallback() {
   logger_->LogSocketEventForMessage(
       channel_id_, proto::MESSAGE_WRITTEN, request.message_namespace,
       base::StringPrintf("Bytes: %d", bytes_consumed));
-  base::ThreadTaskRunnerHandle::Get()->PostTask(FROM_HERE,
-                                                base::Bind(&base::DoNothing));
   base::ThreadTaskRunnerHandle::Get()->PostTask(
       FROM_HERE, base::Bind(request.callback, net::OK));