Example of how to interpose yourself in a message stream.

ipc/ipc_channel_proxy.h

Index: ipc/ipc_channel_proxy.h
===================================================================
--- ipc/ipc_channel_proxy.h	(revision 83739)
+++ ipc/ipc_channel_proxy.h	(working copy)
@@ -99,6 +99,15 @@
     }
   };
 
+  // Interface for a filter to be imposed on outgoing messages which can
+  // re-write the message.  Used mainly for testing.
+  class OutgoingMessageFilter {
+   public:
+    // Returns a re-written message, freeing the original, or simply the
+    // original unchanged if no rewrite indicated.
+    virtual Message *Rewrite(Message *message) = 0;
+  };
+
   // Initializes a channel proxy.  The channel_handle and mode parameters are
   // passed directly to the underlying IPC::Channel.  The listener is called on
   // the thread that creates the ChannelProxy.  The filter's OnMessageReceived
@@ -140,6 +149,10 @@
   void AddFilter(MessageFilter* filter);
   void RemoveFilter(MessageFilter* filter);
 
+  void set_outgoing_message_filter(OutgoingMessageFilter* filter) {
+    outgoing_message_filter_ = filter;
+  }
+
   // Called to clear the pointer to the IPC message loop when it's going away.
   void ClearIPCMessageLoop();
 
@@ -235,6 +248,10 @@
 
   Context* context() { return context_; }
 
+  OutgoingMessageFilter* outgoing_message_filter() {
+    return outgoing_message_filter_;
+  }
+
  private:
   friend class SendTask;
 
@@ -245,6 +262,8 @@
   // can safely be destroyed while the background thread continues to do stuff
   // that involves this data.
   scoped_refptr<Context> context_;
+
+  OutgoingMessageFilter* outgoing_message_filter_;
 };
 
 }  // namespace IPC