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

remoting/host/ipc_session_controller.cc

Index: remoting/host/ipc_session_controller.cc
diff --git a/remoting/host/ipc_session_controller.cc b/remoting/host/ipc_session_controller.cc
new file mode 100644
index 0000000000000000000000000000000000000000..e925038e1836c7d2fb7e7943b4e34cd05eb4a1ba
--- /dev/null
+++ b/remoting/host/ipc_session_controller.cc
@@ -0,0 +1,26 @@
+// 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.
+
+#include "remoting/host/ipc_session_controller.h"
+
+#include "base/logging.h"
+#include "remoting/host/desktop_session_proxy.h"
+
+namespace remoting {
+
+IpcSessionController::IpcSessionController(
+    scoped_refptr<DesktopSessionProxy> desktop_session_proxy)
+    : desktop_session_proxy_(desktop_session_proxy) {
+}
+
+IpcSessionController::~IpcSessionController() {
+}
+
+void IpcSessionController::OnClientResolutionChanged(
+    const SkIPoint& client_dpi,
+    const SkISize& client_size) {
+  NOTIMPLEMENTED();

[Jamie, 2013/03/09 00:53:18]

Can you add a note to the CL description stating that this is just the plumbing
for now and that the resize-to-client feature is not yet implemented for the
multi-process host?

[alexeypa (please no reviews), 2013/03/09 01:05:34]

Done.

+}
+
+}  // namespace remoting