Add field-initializing factory static methods to generated mojo unions.

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

 class {{export_attribute}} {{union.name}} {
  public:
   using DataView = {{union.name}}DataView;
   using Data_ = internal::{{union.name}}_Data;
   using Tag = Data_::{{union.name}}_Tag;
 
-  static {{union.name}}Ptr New();
+  static {{union.name}}Ptr New() {
+    return {{union.name}}Ptr(base::in_place);
+  }
+
+{%-  for field in union.fields %}
+  // Construct an instance holding |{{field.name}}|.
+  static {{union.name}}Ptr
+  New{{field.name|under_to_camel}}(
+      {{field.kind|cpp_wrapper_param_type}} {{field.name}}) {
+    auto result = {{union.name}}Ptr(base::in_place);
+    result->set_{{field.name}}(std::move({{field.name}}));
+    return result;
+  }
+{%- endfor %}
 
   template <typename U>
   static {{union.name}}Ptr From(const U& u) {
     return mojo::TypeConverter<{{union.name}}Ptr, U>::Convert(u);
   }
 
   template <typename U>
   U To() const {
     return mojo::TypeConverter<U, {{union.name}}>::Convert(*this);
   }
@@ skipping 33 matching lines @@
     return *(data_.{{field.name}});
 {%-   else %}
     return data_.{{field.name}};
 {%-   endif %}
   }
 
   void set_{{field.name}}({{field.kind|cpp_wrapper_param_type}} {{field.name}});
 {%- endfor %}
 
  private:
-  friend class mojo::internal::UnionAccessor<{{union.name}}>;
   union Union_ {
     Union_() {}
     ~Union_() {}
 
 {%- for field in union.fields %}
 {%-   if field.kind|is_object_kind or
          field.kind|is_any_handle_or_interface_kind %}
     {{field.kind|cpp_wrapper_type}}* {{field.name}};
 {%-   else %}
     {{field.kind|cpp_wrapper_type}} {{field.name}};
 {%-   endif %}
 {%- endfor %}
   };
-  void SwitchActive(Tag new_active);
-  void SetActive(Tag new_active);
   void DestroyActive();
   Tag tag_;
   Union_ data_;
 };