Allow ImportantFileWriter to take in a pre-write callback.

components/prefs/pref_filter.h

 // Copyright 2013 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.
 
 #ifndef COMPONENTS_PREFS_PREF_FILTER_H_
 #define COMPONENTS_PREFS_PREF_FILTER_H_
 
 #include <memory>
 #include <string>
+#include <utility>
 
 #include "base/callback_forward.h"
 #include "components/prefs/base_prefs_export.h"
 
 namespace base {
 class DictionaryValue;
 class Value;
 }  // namespace base
 
 // Filters preferences as they are loaded from disk or updated at runtime.
 // Currently supported only by JsonPrefStore.
 class COMPONENTS_PREFS_EXPORT PrefFilter {
  public:
+  // A pair of pre-write and post-write callbacks.
+  using OnWriteCallbackPair =
+      std::pair<base::Closure, base::Callback<void(bool success)>>;
+
   // A callback to be invoked when |prefs| have been read (and possibly
   // pre-modified) and are now ready to be handed back to this callback's
   // builder. |schedule_write| indicates whether a write should be immediately
   // scheduled (typically because the |prefs| were pre-modified).
-  typedef base::Callback<void(std::unique_ptr<base::DictionaryValue> prefs,
-                              bool schedule_write)>
-      PostFilterOnLoadCallback;
+  using PostFilterOnLoadCallback =
+      base::Callback<void(std::unique_ptr<base::DictionaryValue> prefs,
+                          bool schedule_write)>;
 
   virtual ~PrefFilter() {}
 
   // This method is given ownership of the |pref_store_contents| read from disk
   // before the underlying PersistentPrefStore gets to use them. It must hand
   // them back via |post_filter_on_load_callback|, but may modify them first.
   // Note: This method is asynchronous, which may make calls like
   // PersistentPrefStore::ReadPrefs() asynchronous. The owner of filtered
   // PersistentPrefStores should handle this to make the reads look synchronous
   // to external users (see SegregatedPrefStore::ReadPrefs() for an example).
   virtual void FilterOnLoad(
       const PostFilterOnLoadCallback& post_filter_on_load_callback,
       std::unique_ptr<base::DictionaryValue> pref_store_contents) = 0;
 
   // Receives notification when a pref store value is changed, before Observers
   // are notified.
   virtual void FilterUpdate(const std::string& path) = 0;
 
   // Receives notification when the pref store is about to serialize data
   // contained in |pref_store_contents| to a string. Modifications to
   // |pref_store_contents| will be persisted to disk and also affect the
   // in-memory state.
-  virtual void FilterSerializeData(
+  // If the returned callbacks are non-null, they will be registered to be
+  // invoked synchronously after the next write (from the I/O TaskRunner so they
+  // must not be bound to thread-unsafe member state).
+  virtual OnWriteCallbackPair FilterSerializeData(
       base::DictionaryValue* pref_store_contents) = 0;
 };
 
 #endif  // COMPONENTS_PREFS_PREF_FILTER_H_