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

remoting/host/desktop_session_proxy.h

Index: remoting/host/desktop_session_proxy.h
diff --git a/remoting/host/desktop_session_proxy.h b/remoting/host/desktop_session_proxy.h
index 927fb4d287077793676c2f89b83e0f4d447966eb..e484e44222d4a65bf4a901c242d5d5847e8c539d 100644
--- a/remoting/host/desktop_session_proxy.h
+++ b/remoting/host/desktop_session_proxy.h
@@ -43,6 +43,7 @@ struct DesktopSessionParams;
 struct DesktopSessionProxyTraits;
 class IpcAudioCapturer;
 class IpcVideoFrameCapturer;
+class SessionController;
 
 // DesktopSessionProxy is created by an owning DesktopEnvironment to route
 // requests from stubs to the DesktopSessionAgent instance through
@@ -75,6 +76,7 @@ class DesktopSessionProxy
   scoped_ptr<EventExecutor> CreateEventExecutor(
       scoped_refptr<base::SingleThreadTaskRunner> input_task_runner,
       scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner);
+  scoped_ptr<SessionController> CreateSessionController();
   scoped_ptr<media::ScreenCapturer> CreateVideoCapturer(
       scoped_refptr<base::SingleThreadTaskRunner> capture_task_runner,
       scoped_refptr<base::SingleThreadTaskRunner> encode_task_runner);