Implement unions as members of structs.

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

 {%- import "validation_macros.tmpl" as validation_macros %}
 {%- set class_name = union.name ~ "_Data" %}
 {%- set enum_name = union.name ~ "_Tag" -%}
 
 // static
 {{class_name}}* {{class_name}}::New(mojo::internal::Buffer* buf) {
   return new (buf->Allocate(sizeof({{class_name}}))) {{class_name}}();
 }
 
 // static
 bool {{class_name}}::Validate(const void* data,
-                              mojo::internal::BoundsChecker* bounds_checker) {
+                              mojo::internal::BoundsChecker* bounds_checker,
+                              bool inlined) {
   if (!data) {
     return true;
   }
 
   if (!mojo::internal::IsAligned(data)) {
     ReportValidationError(mojo::internal::VALIDATION_ERROR_MISALIGNED_OBJECT);
     return false;
   }
 
-  if (!bounds_checker->ClaimMemory(data, sizeof({{class_name}}))) {
+  // If the union is inlined in another structure its memory was already claimed.
+  // This ONLY applies to the union itself, NOT anything which the union points
+  // to.
+  if (!inlined && !bounds_checker->ClaimMemory(data, sizeof({{class_name}}))) {
     ReportValidationError(
         mojo::internal::VALIDATION_ERROR_ILLEGAL_MEMORY_RANGE);
     return false;
   }
   const {{class_name}}* object = static_cast<const {{class_name}}*>(data);
   MOJO_ALLOW_UNUSED_LOCAL(object);
 
   switch (object->tag) {
 {%  for field in union.fields %}
     case {{enum_name}}::{{field.name|upper}}:
 {{ validation_macros.validate_union_field(field, union)|indent(6) }}
 {%- endfor %}
     default:
       ReportValidationError(
           mojo::internal::VALIDATION_ERROR_UNKNOWN_UNION_TAG,
           "unknown tag in {{union.name}}");
       return false;
   }
 }
 
+void {{class_name}}::set_null() {
+  size = 0U;
+  tag = static_cast<{{enum_name}}>(0);
+  data.unknown = 0U;
+}
+
 {{class_name}}::{{class_name}}() {
 }
 
 void {{class_name}}::EncodePointersAndHandles(
     std::vector<mojo::Handle>* handles) {
   // TODO(azani): Implement pointers and handles.
 }
 
 void {{class_name}}::DecodePointersAndHandles(
     std::vector<mojo::Handle>* handles) {
   // TODO(azani): Implement pointers and handles.
 }