Allow ImportantFileWriter to take in a pre-write callback.

components/user_prefs/tracked/tracked_preferences_migration_unittest.cc

Index: components/user_prefs/tracked/tracked_preferences_migration_unittest.cc
diff --git a/components/user_prefs/tracked/tracked_preferences_migration_unittest.cc b/components/user_prefs/tracked/tracked_preferences_migration_unittest.cc
index 3340649fe7d55b85739ff3cf3794d75f58ca5831..b1137543d3c5c128a77454a29e546f48ceb3eb0a 100644
--- a/components/user_prefs/tracked/tracked_preferences_migration_unittest.cc
+++ b/components/user_prefs/tracked/tracked_preferences_migration_unittest.cc
@@ -47,9 +47,11 @@ 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: