Get rid of media_engine_ from BaseChannel; only VoiceChannel needs it.

talk/session/media/channelmanager.cc

 /*
  * libjingle
  * Copyright 2004 Google Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
  *
  *  1. Redistributions of source code must retain the above copyright notice,
  *     this list of conditions and the following disclaimer.
  *  2. Redistributions in binary form must reproduce the above copyright notice,
@@ skipping 404 matching lines @@
   ASSERT(initialized_);
   ASSERT(worker_thread_ == rtc::Thread::Current());
   VideoMediaChannel* media_channel =
       // voice_channel can be NULL in case of NullVoiceEngine.
       media_engine_->CreateVideoChannel(
           options, voice_channel ? voice_channel->media_channel() : NULL);
   if (media_channel == NULL)
     return NULL;
 
   VideoChannel* video_channel = new VideoChannel(
-      worker_thread_, media_engine_.get(), media_channel,
+      worker_thread_, media_channel,
       session, content_name, rtcp);
   if (!video_channel->Init()) {
     delete video_channel;
     return NULL;
   }
   video_channels_.push_back(video_channel);
   return video_channel;
 }
 
 void ChannelManager::DestroyVideoChannel(VideoChannel* video_channel) {
@@ skipping 510 matching lines @@
     const VideoFormat& max_format) {
   device_manager_->SetVideoCaptureDeviceMaxFormat(usb_id, max_format);
 }
 
 bool ChannelManager::StartAecDump(rtc::PlatformFile file) {
   return worker_thread_->Invoke<bool>(
       Bind(&MediaEngineInterface::StartAecDump, media_engine_.get(), file));
 }
 
 }  // namespace cricket