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 83227)
+++ content/common/child_thread.h	(working copy)
@@ -20,6 +20,7 @@
 
 namespace IPC {
 class SyncChannel;
+class OutgoingMessageFilter;
 class SyncMessageFilter;
 }
 
@@ -42,6 +43,9 @@
 
   IPC::Channel::Listener* ResolveRoute(int32 routing_id);
 
+  // Adds a filter to rewrite outgoing messages (used mainly for testing).

[jam, 2011/04/28 23:43:46]

how about just making channel() public, it seems that should have been done from
the start.  that way don't need to add methods to wrap the channel one.

+  void SetOutgoingMessageFilter(IPC::OutgoingMessageFilter* filter);
+
   // Creates a ResourceLoaderBridge.
   // Tests can override this method if they want a custom loading behavior.
   virtual webkit_glue::ResourceLoaderBridge* CreateBridge(