Add support for passing an arbitrary parameter to an IPC message handler. The motivation is for Web…

chrome/browser/spellchecker/spellcheck_message_filter_mac_browsertest.cc

 // Copyright (c) 2012 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 "base/command_line.h"
 #include "base/memory/scoped_vector.h"
 #include "base/message_loop/message_loop.h"
 #include "base/stl_util.h"
 #include "base/strings/utf_string_conversions.h"
 #include "chrome/browser/spellchecker/spellcheck_message_filter_mac.h"
@@ skipping 34 matching lines @@
 
   SpellCheckHostMsg_RequestTextCheck to_be_received(
       123, 456, base::UTF8ToUTF16("zz."), std::vector<SpellCheckMarker>());
   bool handled = false;
   target->OnMessageReceived(to_be_received, &handled);
   EXPECT_TRUE(handled);
 
   base::MessageLoopForUI::current()->Run();
   EXPECT_EQ(1U, target->sent_messages_.size());
 
-  int sent_identifier;
-  std::vector<SpellCheckResult> sent_results;
+  SpellCheckMsg_RespondTextCheck::Param params;
   bool ok = SpellCheckMsg_RespondTextCheck::Read(
-      target->sent_messages_[0], &sent_identifier, &sent_results);
+      target->sent_messages_[0], &params);
+  std::vector<SpellCheckResult> sent_results = params.b;
+ 
   EXPECT_TRUE(ok);
   EXPECT_EQ(1U, sent_results.size());
   EXPECT_EQ(sent_results[0].location, 0);
   EXPECT_EQ(sent_results[0].length, 2);
   EXPECT_EQ(sent_results[0].decoration,
             SpellCheckResult::SPELLING);
 }