Apparently we're supposed to forward the manifest_version attribute to the

chrome/common/extensions/extension_messages.cc

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/common/extensions/extension_messages.h"
 
 #include "chrome/common/extensions/extension_constants.h"
 #include "chrome/common/extensions/manifest.h"
 #include "content/public/common/common_param_traits.h"
 
@@ skipping 23 matching lines @@
       explicit_hosts(extension->GetActivePermissions()->explicit_hosts()),
       scriptable_hosts(extension->GetActivePermissions()->scriptable_hosts()),
       id(extension->id()),
       creation_flags(extension->creation_flags()) {
   // As we need more bits of extension data in the renderer, add more keys to
   // this list.
   const char* kRendererExtensionKeys[] = {
     extension_manifest_keys::kApp,
     extension_manifest_keys::kContentScripts,
     extension_manifest_keys::kIcons,
+    extension_manifest_keys::kManifestVersion,
     extension_manifest_keys::kName,
     extension_manifest_keys::kPageAction,
     extension_manifest_keys::kPageActions,
     extension_manifest_keys::kPermissions,
     extension_manifest_keys::kPlatformApp,
     extension_manifest_keys::kPublicKey,
     extension_manifest_keys::kVersion,
   };
 
   // Copy only the data we need and bypass the manifest type checks.
@@ skipping 135 matching lines @@
          ReadParam(m, iter, &p->explicit_hosts) &&
          ReadParam(m, iter, &p->scriptable_hosts);
 }
 
 void ParamTraits<ExtensionMsg_Loaded_Params>::Log(const param_type& p,
                                                   std::string* l) {
   l->append(p.id);
 }
 
 }  // namespace IPC