Example of how to interpose yourself in a message stream.

content/common/child_thread.cc

Index: content/common/child_thread.cc
===================================================================
--- content/common/child_thread.cc	(revision 83227)
+++ content/common/child_thread.cc	(working copy)
@@ -127,6 +127,10 @@
   return router_.ResolveRoute(routing_id);
 }
 
+void ChildThread::SetOutgoingMessageFilter(IPC::OutgoingMessageFilter* filter){
+  channel_->set_outgoing_message_filter(filter);
+}
+
 webkit_glue::ResourceLoaderBridge* ChildThread::CreateBridge(
     const webkit_glue::ResourceLoaderBridge::RequestInfo& request_info) {
   return resource_dispatcher()->CreateBridge(request_info);