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

remoting/host/session_controller.h

+// Copyright (c) 2013 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_SESSION_CONTROLLER_H_
+#define REMOTING_HOST_SESSION_CONTROLLER_H_
+
+#include "base/basictypes.h"
+#include "third_party/skia/include/core/SkPoint.h"
+#include "third_party/skia/include/core/SkSize.h"
+
+namespace remoting {
+
+class SessionController {
+ public:
+  virtual ~SessionController() {}
+
+  // Called when the client view size or pixel density change.
+  virtual void OnClientResolutionChanged(const SkIPoint& client_dpi,
+                                         const SkISize& client_size) = 0;
+};
+
+}  // namespace remoting
+
+#endif  // REMOTING_HOST_SESSION_CONTROLLER_H_