Mojo C++ bindings: add support for UnionTraits.

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

 {%- set class_name = union.name ~ "_Data" -%}
 {%- set enum_name = union.name ~ "_Tag" -%}
 {%- import "struct_macros.tmpl" as struct_macros %}
 
 class {{class_name}} {
  public:
   // 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() {
+    size = 0U;
+    tag = static_cast<{{enum_name}}>(0);
+    data.unknown = 0U;
   }
 
-  void set_null();
-
   enum class {{enum_name}} : uint32_t {
 {%  for field in union.fields %}
     {{field.name|upper}},
 {%- endfor %}
   };
 
   // A note on layout:
   // "Each non-static data member is allocated as if it were the sole member of
   // a struct." - Section 9.5.2 ISO/IEC 14882:2011 (The C++ Spec)
   union MOJO_ALIGNAS(8) Union_ {
 {%- for field in union.fields %}
 {%-   if field.kind.spec == 'b' %}
     uint8_t f_{{field.name}} : 1;
 {%-   else %}
     {{field.kind|cpp_union_field_type}} f_{{field.name}};
 {%-   endif %}
 {%- endfor %}
     uint64_t unknown;
   };
 
   uint32_t size;
   {{enum_name}} tag;
   Union_ data;
 };
 static_assert(sizeof({{class_name}}) == mojo::internal::kUnionDataSize,
               "Bad sizeof({{class_name}})");