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

remoting/host/ipc_desktop_environment.cc

Index: remoting/host/ipc_desktop_environment.cc
diff --git a/remoting/host/ipc_desktop_environment.cc b/remoting/host/ipc_desktop_environment.cc
index 51ca9b16cd206081dcdbd2d39d8b23424a064218..53f90e1d3cd7a33ed1f0d09a41ccf807e6624086 100644
--- a/remoting/host/ipc_desktop_environment.cc
+++ b/remoting/host/ipc_desktop_environment.cc
@@ -18,6 +18,7 @@
 #include "remoting/host/desktop_session.h"
 #include "remoting/host/desktop_session_proxy.h"
 #include "remoting/host/event_executor.h"
+#include "remoting/host/session_controller.h"
 
 namespace remoting {
 
@@ -59,6 +60,12 @@ scoped_ptr<EventExecutor> IpcDesktopEnvironment::CreateEventExecutor(
                                                      ui_task_runner);
 }
 
+scoped_ptr<SessionController> IpcDesktopEnvironment::CreateSessionController() {
+  DCHECK(caller_task_runner_->BelongsToCurrentThread());
+
+  return desktop_session_proxy_->CreateSessionController();
+}
+
 scoped_ptr<media::ScreenCapturer> IpcDesktopEnvironment::CreateVideoCapturer(
     scoped_refptr<base::SingleThreadTaskRunner> capture_task_runner,
     scoped_refptr<base::SingleThreadTaskRunner> encode_task_runner) {