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

remoting/host/chromoting_host.h

Index: remoting/host/chromoting_host.h
diff --git a/remoting/host/chromoting_host.h b/remoting/host/chromoting_host.h
index 5d9d236048b3adcdf7cd9c161071624d64ccb942..9aa9afd5f3efdb697de0be26f305ce7fee74928e 100644
--- a/remoting/host/chromoting_host.h
+++ b/remoting/host/chromoting_host.h
@@ -126,9 +126,6 @@ class ChromotingHost : public base::RefCountedThreadSafe<ChromotingHost>,
       ClientSession* session,
       const std::string& channel_name,
       const protocol::TransportRoute& route) OVERRIDE;
-  virtual void OnClientResolutionChanged(ClientSession* session,
-                                         const SkISize& size,
-                                         const SkIPoint& dpi) OVERRIDE;
 
   // SessionManager::Listener implementation.
   virtual void OnSessionManagerReady() OVERRIDE;