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 83074)
+++ content/common/child_thread.cc	(working copy)
@@ -15,6 +15,7 @@
 #include "content/common/notification_service.h"
 #include "content/common/quota_dispatcher.h"
 #include "content/common/resource_dispatcher.h"
+#include "content/common/set_ipc_fuzzing.h"
 #include "content/common/socket_stream_dispatcher.h"
 #include "ipc/ipc_logging.h"
 #include "ipc/ipc_sync_channel.h"
@@ -66,6 +67,11 @@
   channel_->AddFilter(new ChildTraceMessageFilter());
 #endif
 
+  if (CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kEnableIPCFuzzing)) {
+    SetIPCFuzzing(channel_.get());

[jam, 2011/04/27 21:58:07]

the code that you added in content should be in chrome layer instead I think,
since content is the bare minimum for rendering a page using a multi-process
browser.  and I think you'll want this in chrome layer anyways in case you want
to do something special with chrome IPCs that the content layer doesn't know
about.

+  }
+
   // When running in unit tests, there is already a NotificationService object.
   // Since only one can exist at a time per thread, check first.
   if (!NotificationService::current())