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

remoting/host/host_mock_objects.cc

Index: remoting/host/host_mock_objects.cc
diff --git a/remoting/host/host_mock_objects.cc b/remoting/host/host_mock_objects.cc
index f8f1062cef7ff131acadd0c49db86dd8a7538225..0dd33e9558efd85c12e058d26a7be90f9cd9f45c 100644
--- a/remoting/host/host_mock_objects.cc
+++ b/remoting/host/host_mock_objects.cc
@@ -34,6 +34,11 @@ scoped_ptr<EventExecutor> MockDesktopEnvironment::CreateEventExecutor(
                                                           ui_task_runner));
 }
 
+scoped_ptr<SessionController>
+MockDesktopEnvironment::CreateSessionController() {
+  return scoped_ptr<SessionController>(CreateSessionControllerPtr());
+}
+
 scoped_ptr<media::ScreenCapturer> MockDesktopEnvironment::CreateVideoCapturer(
     scoped_refptr<base::SingleThreadTaskRunner> capture_task_runner,
     scoped_refptr<base::SingleThreadTaskRunner> encode_task_runner) {
@@ -89,4 +94,8 @@ MockHostStatusObserver::MockHostStatusObserver() {}
 
 MockHostStatusObserver::~MockHostStatusObserver() {}
 
+MockSessionController::MockSessionController() {}
+
+MockSessionController::~MockSessionController() {}
+
 }  // namespace remoting