Add field-initializing constructors to generated mojo structs.

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

 class {{export_attribute}} {{struct.name}} {
  public:
   using DataView = {{struct.name}}DataView;
   using Data_ = internal::{{struct.name}}_Data;
 
 {#--- Enums #}
 {%- for enum in struct.enums -%}
   using {{enum.name}} = {{enum|get_name_for_kind(flatten_nested_kind=True)}};
 {%- endfor %}
 
 {#--- Constants #}
 {%- for constant in struct.constants %}
   static {{constant|format_constant_declaration(nested=True)}};
 {%- endfor %}
 
-  static {{struct.name}}Ptr New();
+  template <typename... Args>
+  static {{struct.name}}Ptr New(Args&&... args) {
+    return {{struct.name}}Ptr(
+        base::in_place,
+        std::forward<Args>(args)...);
+  }
 
   template <typename U>
   static {{struct.name}}Ptr From(const U& u) {
     return mojo::TypeConverter<{{struct.name}}Ptr, U>::Convert(u);
   }
 
   template <typename U>
   U To() const {
     return mojo::TypeConverter<U, {{struct.name}}>::Convert(*this);
   }
 
-  {{struct.name}}();
+{% for constructor in struct|struct_constructors %}
+  {% if constructor.params|length == 1 %}explicit {% endif %}{{struct.name}}(
+{%-   for field in constructor.params %}
+{%-     set type = field.kind|cpp_wrapper_param_type %}
+{%-     set name = field.name %}
+      {{type}} {{name}}
+{%-     if not loop.last -%},{%- endif %}
+{%-   endfor %});
+{% endfor %}
   ~{{struct.name}}();
 
   // Clone() is a template so it is only instantiated if it is used. Thus, the
   // bindings generator does not need to know whether Clone() or copy
   // constructor/assignment are available for members.
   template <typename StructPtrType = {{struct.name}}Ptr>
   {{struct.name}}Ptr Clone() const;
 
   // Equals() is a template so it is only instantiated if it is used. Thus, the
   // bindings generator does not need to know whether Equals() or == operator
@@ skipping 30 matching lines @@
 {%-   set name = field.name %}
   {{type}} {{name}};
 {%- endfor %}
 
 {%- if struct|contains_move_only_members %}
  private:
   DISALLOW_COPY_AND_ASSIGN({{struct.name}});
 {%- endif %}
 };