Implement unions as members of structs.

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

Index: mojo/public/tools/bindings/generators/cpp_templates/union_declaration.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/union_declaration.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/union_declaration.tmpl
index 6e6fe95a74c1dc207af7f5d252494a1936fd7e14..bc77c112dd0f010dbd0d4f817d648c4e0031c4f4 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/union_declaration.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/union_declaration.tmpl
@@ -4,10 +4,22 @@
 
 class {{class_name}} {
  public:
+  // Used to identify Mojom Union Data Classes.
+  typedef void MojomUnionDataType;
   static {{class_name}}* New(mojo::internal::Buffer* buf);
+  {{class_name}}();
+  // Do nothing in the destructor since it won't be called.
+  ~{{class_name}}() {}
 
   static bool Validate(const void* data,
-                       mojo::internal::BoundsChecker* bounds_checker);
+                       mojo::internal::BoundsChecker* bounds_checker,
+                       bool inlined);
+
+  bool is_null() const {
+    return size == 0;
+  }
+
+  void set_null();
 
   enum class {{enum_name}} : uint32_t {
 {%  for field in union.fields %}
@@ -33,16 +45,12 @@ class {{class_name}} {
     uint64_t unknown;
   };
 
-  uint32_t reserved;
+  uint32_t size;
   {{enum_name}} tag;
   Union_ data;
 
   void EncodePointersAndHandles(std::vector<mojo::Handle>* handles);
   void DecodePointersAndHandles(std::vector<mojo::Handle>* handles);
- 
- private:
-  {{class_name}}();
-  ~{{class_name}}() = delete;
 };
 static_assert(sizeof({{class_name}}) == 16,
               "Bad sizeof({{class_name}})");