Don't make classes derive from ChildProcessHost, and instead have them use it through composition...

content/browser/plugin_process_host.h

Index: content/browser/plugin_process_host.h
===================================================================
--- content/browser/plugin_process_host.h	(revision 112597)
+++ content/browser/plugin_process_host.h	(working copy)
@@ -17,6 +17,7 @@
 #include "base/memory/ref_counted.h"
 #include "content/browser/browser_child_process_host.h"
 #include "content/common/content_export.h"
+#include "ipc/ipc_channel_proxy.h"
 #include "webkit/plugins/webplugininfo.h"
 #include "ui/gfx/native_widget_types.h"
 
@@ -69,7 +70,7 @@
   bool Init(const webkit::WebPluginInfo& info, const std::string& locale);
 
   // Force the plugin process to shutdown (cleanly).
-  virtual void ForceShutdown() OVERRIDE;
+  void ForceShutdown();
 
   virtual bool OnMessageReceived(const IPC::Message& msg) OVERRIDE;
   virtual void OnChannelConnected(int32 peer_pid) OVERRIDE;
@@ -109,6 +110,9 @@
   void AddWindow(HWND window);
 #endif
 
+  // Adds an IPC message filter.  A reference will be kept to the filter.
+  void AddFilter(IPC::ChannelProxy::MessageFilter* filter);
+
  private:
   // Sends a message to the plugin process to request creation of a new channel
   // for the given mime type.