Allow ImportantFileWriter to take in a pre-write callback.

components/user_prefs/tracked/tracked_preferences_migration_unittest.cc

 // Copyright 2014 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 "components/user_prefs/tracked/tracked_preferences_migration.h"
 
 #include <memory>
 #include <set>
 #include <string>
 #include <utility>
@@ skipping 29 matching lines @@
 const char kProtectedPrefValue[] = "protected_value";
 const char kPreviouslyUnprotectedPrefValue[] = "previously_unprotected_value";
 const char kPreviouslyProtectedPrefValue[] = "previously_protected_value";
 
 // A simple InterceptablePrefFilter which doesn't do anything but hand the prefs
 // back downstream in FinalizeFilterOnLoad.
 class SimpleInterceptablePrefFilter : public InterceptablePrefFilter {
  public:
   // PrefFilter remaining implementation.
   void FilterUpdate(const std::string& path) override { ADD_FAILURE(); }
-  void FilterSerializeData(
+  OnWriteCallbackPair FilterSerializeData(
       base::DictionaryValue* pref_store_contents) override {
     ADD_FAILURE();
+    return std::make_pair(base::Closure(),
+                          base::Callback<void(bool success)>());
   }
 
  private:
   // InterceptablePrefFilter implementation.
   void FinalizeFilterOnLoad(
       const PostFilterOnLoadCallback& post_filter_on_load_callback,
       std::unique_ptr<base::DictionaryValue> pref_store_contents,
       bool prefs_altered) override {
     post_filter_on_load_callback.Run(std::move(pref_store_contents),
                                      prefs_altered);
@@ skipping 581 matching lines @@
                        expected_unprotected_values);
 
     base::StringPairs expected_protected_values;
     expected_protected_values.push_back(std::make_pair(
         kProtectedPref, kProtectedPrefValue));
     expected_protected_values.push_back(std::make_pair(
         kPreviouslyUnprotectedPref, kPreviouslyUnprotectedPrefValue));
     VerifyValuesStored(MOCK_PROTECTED_PREF_STORE, expected_protected_values);
   }
 }