Mojo C++ bindings: switch the existing usage of StructTraits to use the new data view interface.

mojo/public/cpp/bindings/lib/string_serialization.h

Index: mojo/public/cpp/bindings/lib/string_serialization.h
diff --git a/mojo/public/cpp/bindings/lib/string_serialization.h b/mojo/public/cpp/bindings/lib/string_serialization.h
index 98c96c0532c44e04446d919494f9050a32123e56..5e65891ef5e0594e0d93a9a0f119f82aba168ecf 100644
--- a/mojo/public/cpp/bindings/lib/string_serialization.h
+++ b/mojo/public/cpp/bindings/lib/string_serialization.h
@@ -58,6 +58,8 @@ struct Serializer<String, MaybeConstUserType> {
   static bool Deserialize(String_Data* input,
                           UserType* output,
                           SerializationContext* context) {
+    if (!input)
+      return CallSetToNullIfExists<Traits>(output);
     return Traits::Read(StringDataView(input), output);
   }
 };