Removing ERROR message for something that's expected to occur.

This error occurred if you created a data channel before there's a data
m= section. But this is expected to happen, since creating a data
channel is how you get a data m= section in the first place.

BUG=chromium:579430
NOTRY=true
TBR=pthatcher@webrtc.org

Review-Url: https://codereview.webrtc.org/2396013002
Cr-Commit-Position: refs/heads/master@{#14537}
diff --git a/webrtc/api/webrtcsession.cc b/webrtc/api/webrtcsession.cc
index df4a48a..ded3023 100644
--- a/webrtc/api/webrtcsession.cc
+++ b/webrtc/api/webrtcsession.cc
@@ -1274,7 +1274,9 @@
 
 bool WebRtcSession::ConnectDataChannel(DataChannel* webrtc_data_channel) {
   if (!data_channel_) {
-    LOG(LS_ERROR) << "ConnectDataChannel called when data_channel_ is NULL.";
+    // Don't log an error here, because DataChannels are expected to call
+    // ConnectDataChannel in this state. It's the only way to initially tell
+    // whether or not the underlying transport is ready.
     return false;
   }
   data_channel_->SignalReadyToSendData.connect(webrtc_data_channel,