Mojo C++ bindings: add support for UnionTraits.

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 d65854073187bfc6aed181e11d6a8d55407820b5..005ba76b611fa0f1bc7cae6af20fa5cbc1413b82 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/union_declaration.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/union_declaration.tmpl
@@ -7,23 +7,26 @@ class {{class_name}} {
   // Used to identify Mojom Union Data Classes.
   typedef void MojomUnionDataType;
 
+  {{class_name}}() {}
+  // Do nothing in the destructor since it won't be called when it is a
+  // non-inlined union.
+  ~{{class_name}}() {}
+
   static {{class_name}}* New(mojo::internal::Buffer* buf) {
     return new (buf->Allocate(sizeof({{class_name}}))) {{class_name}}();
   }
 
-  {{class_name}}();
-  // Do nothing in the destructor since it won't be called.
-  ~{{class_name}}() {}
-
   static bool Validate(const void* data,
                        mojo::internal::ValidationContext* validation_context,
                        bool inlined);
 
-  bool is_null() const {
-    return size == 0;
-  }
+  bool is_null() const { return size == 0; }
 
-  void set_null();
+  void set_null() {
+    size = 0U;
+    tag = static_cast<{{enum_name}}>(0);
+    data.unknown = 0U;
+  }
 
   enum class {{enum_name}} : uint32_t {
 {%  for field in union.fields %}