ResizingHostObserver is created by the desktop environment together with other stubs.

remoting/host/desktop_session_proxy.cc

Index: remoting/host/desktop_session_proxy.cc
diff --git a/remoting/host/desktop_session_proxy.cc b/remoting/host/desktop_session_proxy.cc
index abd3ef297807ef61f4a93c9fe9283cd1d20067ee..1e3ec991ff60467c34b8d25534b730007bb9c7c2 100644
--- a/remoting/host/desktop_session_proxy.cc
+++ b/remoting/host/desktop_session_proxy.cc
@@ -17,7 +17,9 @@
 #include "remoting/host/desktop_session_connector.h"
 #include "remoting/host/ipc_audio_capturer.h"
 #include "remoting/host/ipc_event_executor.h"
+#include "remoting/host/ipc_session_controller.h"
 #include "remoting/host/ipc_video_frame_capturer.h"
+#include "remoting/host/session_controller.h"
 #include "remoting/proto/audio.pb.h"
 #include "remoting/proto/control.pb.h"
 #include "remoting/proto/event.pb.h"
@@ -61,6 +63,10 @@ scoped_ptr<EventExecutor> DesktopSessionProxy::CreateEventExecutor(
   return scoped_ptr<EventExecutor>(new IpcEventExecutor(this));
 }
 
+scoped_ptr<SessionController> DesktopSessionProxy::CreateSessionController() {
+  return scoped_ptr<SessionController>(new IpcSessionController(this));
+}
+
 scoped_ptr<media::ScreenCapturer> DesktopSessionProxy::CreateVideoCapturer(
     scoped_refptr<base::SingleThreadTaskRunner> capture_task_runner,
     scoped_refptr<base::SingleThreadTaskRunner> encode_task_runner) {