Add field-initializing constructors to generated mojo structs.

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

 // static
 {{union.name}}Ptr {{union.name}}::New() {
-  {{union.name}}Ptr rv;
-  mojo::internal::StructHelper<{{union.name}}>::Initialize(&rv);
-  return rv;
+  return {{union.name}}Ptr(base::in_place);
 }
 
 {{union.name}}::{{union.name}}() {
   // TODO(azani): Implement default values here when/if we support them.
   // TODO(azani): Set to UNKNOWN when unknown is implemented.
   SetActive(static_cast<Tag>(0));
 }
 
 {{union.name}}::~{{union.name}}() {
   DestroyActive();
@@ skipping 62 matching lines @@
       return mojo::internal::Hash(seed, data_.{{field.name}});
 {%-   endif %}
 {%- endfor %}
     default:
       NOTREACHED();
       return seed;
   }
 }
 
 {%- endif %}