Pref service: Add a ScopedDictionaryPrefUpdate to track value changes.

mojo/common/values_struct_traits.cc

 // Copyright 2016 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/memory/ptr_util.h"
 #include "mojo/common/values_struct_traits.h"
 
 namespace mojo {
 
 bool StructTraits<common::mojom::ListValueDataView,
@@ skipping 30 matching lines @@
 
     dictionary_value->SetWithoutPathExpansion(key, std::move(value));
   }
   *value_out = std::move(dictionary_value);
   return true;
 }
 
 std::unique_ptr<base::DictionaryValue>
 CloneTraits<std::unique_ptr<base::DictionaryValue>, false>::Clone(
     const std::unique_ptr<base::DictionaryValue>& input) {
-  auto result = base::MakeUnique<base::DictionaryValue>();
-  result->MergeDictionary(input.get());
-  return result;
+  return input ? input->CreateDeepCopy() : nullptr;
 }
 
 bool UnionTraits<common::mojom::ValueDataView, std::unique_ptr<base::Value>>::
     Read(common::mojom::ValueDataView data,
          std::unique_ptr<base::Value>* value_out) {
   switch (data.tag()) {
     case common::mojom::ValueDataView::Tag::NULL_VALUE: {
       *value_out = base::MakeUnique<base::Value>();
       return true;
     }
@@ skipping 35 matching lines @@
       std::unique_ptr<base::ListValue> list_value;
       if (!data.ReadListValue(&list_value))
         return false;
       *value_out = std::move(list_value);
       return true;
     }
   }
   return false;
 }
 
+std::unique_ptr<base::Value>
+CloneTraits<std::unique_ptr<base::Value>, false>::Clone(
+    const std::unique_ptr<base::Value>& input) {
+  return input ? input->CreateDeepCopy() : nullptr;
+}
+
 }  // namespace mojo