Add field-initializing factory static methods to generated mojo unions.

mojo/public/tools/bindings/generators/cpp_templates/union_traits_definition.tmpl

 {%- set mojom_type = union|get_qualified_name_for_kind %}
 
 // static
 bool UnionTraits<{{mojom_type}}::DataView, {{mojom_type}}Ptr>::Read(
     {{mojom_type}}::DataView input,
     {{mojom_type}}Ptr* output) {
-  *output = {{mojom_type}}::New();
-  {{mojom_type}}Ptr& result = *output;
+  using UnionType = {{mojom_type}};
+  using Tag = UnionType::Tag;
 
-  internal::UnionAccessor<{{mojom_type}}> result_acc(result.get());
   switch (input.tag()) {
 {%- for field in union.fields %}
-    case {{mojom_type}}::Tag::{{field.name|upper}}: {
+    case Tag::{{field.name|upper}}: {
 {%-   set name = field.name %}
 {%-   set kind = field.kind %}
 {%-   set serializer_type = kind|unmapped_type_for_serializer %}
 {%-   if kind|is_object_kind %}
-      result_acc.SwitchActive({{mojom_type}}::Tag::{{name|upper}});
-      if (!input.Read{{name|under_to_camel}}(result_acc.data()->{{name}}))
+      auto result = UnionType::New(UnionType::{{field.name}});
+      if (!input.Read{{name|under_to_camel}}(&result->get_{{name}}()))
         return false;
 
+      *output = std::move(result);
 {%-   elif kind|is_any_handle_kind %}
-      auto result_{{name}} = input.Take{{name|under_to_camel}}();
-      result->set_{{name}}(std::move(result_{{name}}));
+      *output = UnionType::New(UnionType::{{field.name}}, input.Take{{name|under_to_camel}}());
 
 {%-   elif kind|is_any_interface_kind %}
       auto result_{{name}} =
-          input.Take{{name|under_to_camel}}<typename std::remove_reference<decltype(result->get_{{name}}())>::type>();
-      result->set_{{name}}(std::move(result_{{name}}));
+          input.Take{{name|under_to_camel}}<typename std::remove_reference<decltype((*output)->get_{{name}}())>::type>();
+      *output = UnionType::New(UnionType::{{field.name}}, std::move(result_{{name}}));
 
 {%-   elif kind|is_enum_kind %}
-      decltype(result->get_{{name}}()) result_{{name}};
+      decltype((*output)->get_{{name}}()) result_{{name}};
       if (!input.Read{{name|under_to_camel}}(&result_{{name}}))
         return false;
-      result->set_{{name}}(result_{{name}});
+      *output = UnionType::New(UnionType::{{field.name}}, result_{{name}});
 
 {%-   else %}
-      result->set_{{name}}(input.{{name}}());
+      *output = UnionType::New(UnionType::{{field.name}}, input.{{name}}());
 {%-   endif %}
       break;
     }
 {%- endfor %}
     default:
       return false;
   }
   return true;
 }