Fix DesktopSessioAgent crash when handling nullptr OnCaptureCompleted callback.

remoting/host/desktop_session_agent.h

Index: remoting/host/desktop_session_agent.h
diff --git a/remoting/host/desktop_session_agent.h b/remoting/host/desktop_session_agent.h
index c965d37129379f0e729ef72dc9ccc671fc6c9524..554d27f27f08a29e12a136dafa742aa9728efaf7 100644
--- a/remoting/host/desktop_session_agent.h
+++ b/remoting/host/desktop_session_agent.h
@@ -182,9 +182,6 @@ class DesktopSessionAgent
   // the network process connects to the pipe.
   base::File desktop_pipe_;
 
-  // Size of the most recent captured video frame.
-  webrtc::DesktopSize current_size_;
-
   // True if the desktop session agent has been started.
   bool started_ = false;