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

remoting/host/desktop_environment.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef REMOTING_HOST_DESKTOP_ENVIRONMENT_H_
 #define REMOTING_HOST_DESKTOP_ENVIRONMENT_H_
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/callback_forward.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 
 namespace base {
 class SingleThreadTaskRunner;
 }  // namespace base
 
 namespace media {
 class ScreenCapturer;
 }  // namespace media
 
 namespace remoting {
 
 class AudioCapturer;
 class EventExecutor;
+class SessionController;
 
 // Provides factory methods for creation of audio/video capturers and event
 // executor for a given desktop environment.
 class DesktopEnvironment {
  public:
   virtual ~DesktopEnvironment() {}
 
-  // Factory methods used to create audio/video capturers and event executor for
-  // a particular desktop environment.
+  // Factory methods used to create audio/video capturers, event executor, and
+  // session controller for a particular desktop environment.
   virtual scoped_ptr<AudioCapturer> CreateAudioCapturer(
       scoped_refptr<base::SingleThreadTaskRunner> audio_task_runner) = 0;
   virtual scoped_ptr<EventExecutor> CreateEventExecutor(
       scoped_refptr<base::SingleThreadTaskRunner> input_task_runner,
       scoped_refptr<base::SingleThreadTaskRunner> ui_task_runner) = 0;
+  virtual scoped_ptr<SessionController> CreateSessionController() = 0;
   virtual scoped_ptr<media::ScreenCapturer> CreateVideoCapturer(
       scoped_refptr<base::SingleThreadTaskRunner> capture_task_runner,
       scoped_refptr<base::SingleThreadTaskRunner> encode_task_runner) = 0;
 };
 
 // Used to create |DesktopEnvironment| instances.
 class DesktopEnvironmentFactory {
  public:
   virtual ~DesktopEnvironmentFactory() {}
 
   // Creates an instance of |DesktopEnvironment|. |disconnect_callback| may be
   // used by the DesktopEnvironment to disconnect the client session.
   virtual scoped_ptr<DesktopEnvironment> Create(
       const std::string& client_jid,
       const base::Closure& disconnect_callback) = 0;
 
   // Returns |true| if created |DesktopEnvironment| instances support audio
   // capture.
   virtual bool SupportsAudioCapture() const = 0;
 };
 
 }  // namespace remoting
 
 #endif  // REMOTING_HOST_DESKTOP_ENVIRONMENT_H_