Parse messages.json in ExtensionUnpacker (like we do for manifest)....

chrome/common/render_messages_internal.h

 // Copyright (c) 2009 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.
 
 // This header is meant to be included in multiple passes, hence no traditional
 // header guard.
 // See ipc_message_macros.h for explanation of the macros and passes.
 
 #include <map>
 #include <string>
@@ skipping 1893 matching lines @@
   IPC_SYNC_MESSAGE_CONTROL1_1(ViewHostMsg_GetFileSize,
                               FilePath /* path */,
                               int64 /* result */)
 
   //---------------------------------------------------------------------------
   // Utility process host messages:
   // These are messages from the utility process to the browser.  They're here
   // because we ran out of spare message types.
 
   // Reply when the utility process is done unpacking an extension.  |manifest|
-  // is the parsed manifest.json file.  The unpacker should also have written
-  // out a file containing decoded images from the extension.  See
-  // ExtensionUnpacker for details.
-  IPC_MESSAGE_CONTROL1(UtilityHostMsg_UnpackExtension_Succeeded,
-                       DictionaryValue /* manifest */)
+  // is the parsed manifest.json file.  |catalogs| is the list of all parsed
+  // message catalogs and relative paths to them.
+  // The unpacker should also have written out a file containing decoded images
+  // from the extension.  See ExtensionUnpacker for details.
+  IPC_MESSAGE_CONTROL2(UtilityHostMsg_UnpackExtension_Succeeded,
+                       DictionaryValue /* manifest */,
+                       DictionaryValue /* catalogs */)
 
   // Reply when the utility process has failed while unpacking an extension.
   // |error_message| is a user-displayable explanation of what went wrong.
   IPC_MESSAGE_CONTROL1(UtilityHostMsg_UnpackExtension_Failed,
                        std::string /* error_message, if any */)
 
   // Reply when the utility process is done unpacking and parsing JSON data
   // from a web resource.
   IPC_MESSAGE_CONTROL1(UtilityHostMsg_UnpackWebResource_Succeeded,
                        DictionaryValue /* json data */)
@@ skipping 105 matching lines @@
                               std::string /* signed public key and challenge */)
 
 #if defined(SPELLCHECKER_IN_RENDERER)
   // The renderer has tried to spell check a word, but couldn't because no
   // dictionary was available to load. Request that the browser find an
   // appropriate dictionary and return it.
   IPC_MESSAGE_CONTROL0(ViewHostMsg_SpellChecker_RequestDictionary)
 #endif
 
 IPC_END_MESSAGES(ViewHost)