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

remoting/host/client_session.h

Index: remoting/host/client_session.h
diff --git a/remoting/host/client_session.h b/remoting/host/client_session.h
index 5ed34f30ad0099df70cc151110ca97a8d8548592..b6d3a6a2f06794572857057da719553745ae8687 100644
--- a/remoting/host/client_session.h
+++ b/remoting/host/client_session.h
@@ -38,6 +38,7 @@ struct ClientSessionTraits;
 class DesktopEnvironment;
 class DesktopEnvironmentFactory;
 class EventExecutor;
+class SessionController;
 class VideoEncoder;
 class VideoScheduler;
 
@@ -78,12 +79,6 @@ class ClientSession
         const std::string& channel_name,
         const protocol::TransportRoute& route) = 0;
 
-    // Called when the initial client resolution is received, and when it
-    // changes.
-    virtual void OnClientResolutionChanged(ClientSession* client,
-                                           const SkISize& size,
-                                           const SkIPoint& dpi) = 0;
-
    protected:
     virtual ~EventHandler() {}
   };
@@ -230,6 +225,8 @@ class ClientSession
 
   scoped_ptr<EventExecutor> event_executor_;
 
+  scoped_ptr<SessionController> session_controller_;
+
   DISALLOW_COPY_AND_ASSIGN(ClientSession);
 };