Example of how to interpose yourself in a message stream.

content/common/child_thread.h

Index: content/common/child_thread.h
===================================================================
--- content/common/child_thread.h	(revision 83739)
+++ content/common/child_thread.h	(working copy)
@@ -42,6 +42,8 @@
 
   IPC::Channel::Listener* ResolveRoute(int32 routing_id);
 
+  IPC::SyncChannel* channel() { return channel_.get(); }
+
   // Creates a ResourceLoaderBridge.
   // Tests can override this method if they want a custom loading behavior.
   virtual webkit_glue::ResourceLoaderBridge* CreateBridge(
@@ -84,8 +86,6 @@
   virtual void OnSetIPCLoggingEnabled(bool enable);
 #endif
 
-  IPC::SyncChannel* channel() { return channel_.get(); }
-
   void set_on_channel_error_called(bool on_channel_error_called) {
     on_channel_error_called_ = on_channel_error_called;
   }