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

remoting/host/basic_desktop_environment.cc

Index: remoting/host/basic_desktop_environment.cc
diff --git a/remoting/host/basic_desktop_environment.cc b/remoting/host/basic_desktop_environment.cc
index d49de5cfb435bbec88618787908b3703393c7167..99ddca0ba5a797f3072de2925832d339ca64f191 100644
--- a/remoting/host/basic_desktop_environment.cc
+++ b/remoting/host/basic_desktop_environment.cc
@@ -7,7 +7,9 @@
 #include "base/logging.h"
 #include "media/video/capture/screen/screen_capturer.h"
 #include "remoting/host/audio_capturer.h"
+#include "remoting/host/desktop_resizer.h"
 #include "remoting/host/event_executor.h"
+#include "remoting/host/resizing_host_observer.h"
 
 namespace remoting {
 
@@ -34,6 +36,15 @@ scoped_ptr<EventExecutor> BasicDesktopEnvironment::CreateEventExecutor(
   return EventExecutor::Create(input_task_runner, ui_task_runner);
 }
 
+scoped_ptr<SessionController>
+BasicDesktopEnvironment::CreateSessionController() {
+  DCHECK(CalledOnValidThread());
+
+  scoped_ptr<SessionController> session_controller(
+      new ResizingHostObserver(DesktopResizer::Create()));
+  return session_controller.Pass();
+}
+
 scoped_ptr<media::ScreenCapturer> BasicDesktopEnvironment::CreateVideoCapturer(
     scoped_refptr<base::SingleThreadTaskRunner> capture_task_runner,
     scoped_refptr<base::SingleThreadTaskRunner> encode_task_runner) {