Pref service: Add a ScopedDictionaryPrefUpdate to track value changes.

mojo/common/values_struct_traits.cc

Index: mojo/common/values_struct_traits.cc
diff --git a/mojo/common/values_struct_traits.cc b/mojo/common/values_struct_traits.cc
index 6d392ecabdeba15e8875490d259ba67262dcd59e..12e5173e5eadbf1854d3b4a6531f9ea4d9bbb972 100644
--- a/mojo/common/values_struct_traits.cc
+++ b/mojo/common/values_struct_traits.cc
@@ -48,9 +48,7 @@ bool StructTraits<common::mojom::DictionaryValueDataView,
 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>>::
@@ -106,4 +104,10 @@ bool UnionTraits<common::mojom::ValueDataView, std::unique_ptr<base::Value>>::
   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