iwyu: Use callback_old.h where appropriate, part 2.

base/callback_unittest.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 "base/callback.h"
 #include "base/callback_internal.h"
+#include "base/callback_old.h"
 #include "base/memory/scoped_ptr.h"
 
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace base {
 namespace {
 
 class HelperObject {
  public:
   HelperObject() : next_number_(0) { }
@@ skipping 104 matching lines @@
   ASSERT_FALSE(callback_a_.Equals(null_callback_));
 
   callback_a_.Reset();
 
   EXPECT_TRUE(callback_a_.is_null());
   EXPECT_TRUE(callback_a_.Equals(null_callback_));
 }
 
 }  // namespace
 }  // namespace base