Pref service: Add a ScopedDictionaryPrefUpdate to track value changes.

services/preferences/public/cpp/pref_store_impl.cc

 // Copyright 2017 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 "services/preferences/public/cpp/pref_store_impl.h"
 
 #include <memory>
 #include <unordered_set>
+#include <utility>
 
 #include "base/stl_util.h"
 #include "base/values.h"
 
 namespace prefs {
 
 class PrefStoreImpl::Observer {
  public:
   Observer(mojom::PrefStoreObserverPtr observer,
            std::unordered_set<std::string> prefs)
       : observer_(std::move(observer)), prefs_(std::move(prefs)) {}
 
   void OnInitializationCompleted(bool succeeded) {
     observer_->OnInitializationCompleted(succeeded);
   }
 
   void OnPrefChanged(const std::string& key, const base::Value& value) const {
     if (!base::ContainsKey(prefs_, key))
       return;
 
     std::vector<mojom::PrefUpdatePtr> updates;
-    updates.push_back(mojom::PrefUpdate::New(key, value.CreateDeepCopy(), 0));
+    updates.push_back(mojom::PrefUpdate::New(
+        key, mojom::PrefUpdateValue::NewAtomicUpdate(value.CreateDeepCopy()),
+        0));
     observer_->OnPrefsChanged(std::move(updates));
   }
 
   void OnPrefRemoved(const std::string& key) const {
     if (!base::ContainsKey(prefs_, key))
       return;
 
     std::vector<mojom::PrefUpdatePtr> updates;
-    updates.push_back(mojom::PrefUpdate::New(key, nullptr, 0));
+    updates.push_back(mojom::PrefUpdate::New(
+        key, mojom::PrefUpdateValue::NewAtomicUpdate(nullptr), 0));
     observer_->OnPrefsChanged(std::move(updates));
   }
 
  private:
   mojom::PrefStoreObserverPtr observer_;
   const std::unordered_set<std::string> prefs_;
 
   DISALLOW_COPY_AND_ASSIGN(Observer);
 };
 
@@ skipping 54 matching lines @@
   observers_.push_back(base::MakeUnique<Observer>(
       std::move(observer_ptr),
       std::unordered_set<std::string>(prefs_to_observe.begin(),
                                       prefs_to_observe.end())));
   callback.Run(mojom::PrefStoreConnection::New(
       std::move(request), backing_pref_store_->GetValues(),
       backing_pref_store_->IsInitializationComplete()));
 }
 
 }  // namespace prefs