[Cast] Cleanup: Remove TransportXXXXXSender, an unnecessary layer of indirection.

media/cast/transport/cast_transport_sender_impl.cc

Index: media/cast/transport/cast_transport_sender_impl.cc
diff --git a/media/cast/transport/cast_transport_sender_impl.cc b/media/cast/transport/cast_transport_sender_impl.cc
index 8f07832d1c73851e4e9655397754a9fe82dbccab..d80a4e41e6e3e7ba76b3e34a4f4c6dc55c7cec69 100644
--- a/media/cast/transport/cast_transport_sender_impl.cc
+++ b/media/cast/transport/cast_transport_sender_impl.cc
@@ -56,6 +56,7 @@ CastTransportSenderImpl::CastTransportSenderImpl(
              transport_task_runner),
       rtcp_builder_(&pacer_),
       raw_events_callback_(raw_events_callback) {
+  DCHECK(clock_);
   if (!raw_events_callback_.is_null()) {
     DCHECK(raw_events_callback_interval > base::TimeDelta());
     event_subscriber_.reset(new SimpleEventSubscriber);
@@ -74,24 +75,26 @@ CastTransportSenderImpl::~CastTransportSenderImpl() {
 
 void CastTransportSenderImpl::InitializeAudio(
     const CastTransportAudioConfig& config) {
-  pacer_.RegisterAudioSsrc(config.rtp.config.ssrc);
-  audio_sender_.reset(new TransportAudioSender(
-      config, clock_, transport_task_runner_, &pacer_));
-  if (audio_sender_->initialized())
+  audio_sender_.reset(new RtpSender(clock_, transport_task_runner_, &pacer_));
+  if (audio_sender_->InitializeAudio(config)) {
+    pacer_.RegisterAudioSsrc(config.rtp.config.ssrc);
     status_callback_.Run(TRANSPORT_AUDIO_INITIALIZED);
-  else
+  } else {
+    audio_sender_.reset();
     status_callback_.Run(TRANSPORT_AUDIO_UNINITIALIZED);
+  }
 }
 
 void CastTransportSenderImpl::InitializeVideo(
     const CastTransportVideoConfig& config) {
-  pacer_.RegisterVideoSsrc(config.rtp.config.ssrc);
-  video_sender_.reset(new TransportVideoSender(
-      config, clock_, transport_task_runner_, &pacer_));
-  if (video_sender_->initialized())
+  video_sender_.reset(new RtpSender(clock_, transport_task_runner_, &pacer_));
+  if (video_sender_->InitializeVideo(config)) {
+    pacer_.RegisterVideoSsrc(config.rtp.config.ssrc);
     status_callback_.Run(TRANSPORT_VIDEO_INITIALIZED);
-  else
+  } else {
+    video_sender_.reset();
     status_callback_.Run(TRANSPORT_VIDEO_UNINITIALIZED);
+  }
 }
 
 void CastTransportSenderImpl::SetPacketReceiver(