Properly remove the bitrate observer when ViEEncoder is destructed.

BUG=1090

Review URL: https://webrtc-codereview.appspot.com/969013

git-svn-id: http://webrtc.googlecode.com/svn/trunk/webrtc@3196 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/video_engine/vie_encoder.cc b/video_engine/vie_encoder.cc
index 36ad862..b63cdbd 100644
--- a/video_engine/vie_encoder.cc
+++ b/video_engine/vie_encoder.cc
@@ -210,6 +210,9 @@
   WEBRTC_TRACE(webrtc::kTraceMemory, webrtc::kTraceVideo,
                ViEId(engine_id_, channel_id_),
                "ViEEncoder Destructor 0x%p, engine_id: %d", this, engine_id_);
+  if (bitrate_controller_) {
+    bitrate_controller_->RemoveBitrateObserver(bitrate_observer_.get());
+  }
   module_process_thread_.DeRegisterModule(&vcm_);
   module_process_thread_.DeRegisterModule(&vpm_);
   module_process_thread_.DeRegisterModule(default_rtp_rtcp_.get());