Make mojom-generated C++ string constants really constant.

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 %}
-{%-   if constant.kind|is_integral_kind %}
-  static const {{constant.kind|cpp_pod_type}} {{constant.name}} = {{constant|constant_value}};
-{%-   else %}
-  static const {{constant.kind|cpp_pod_type}} {{constant.name}};
-{%-   endif %}
+  static {{constant|format_constant_declaration(nested=True)}};
 {%- endfor %}
 
   static {{struct.name}}Ptr New();
 
   template <typename U>
   static {{struct.name}}Ptr From(const U& u) {
     return mojo::TypeConverter<{{struct.name}}Ptr, U>::Convert(u);
   }
 
   template <typename U>
@@ skipping 45 matching lines @@
 {%-   set name = field.name %}
   {{type}} {{name}};
 {%- endfor %}
 
 {%- if struct|contains_move_only_members %}
  private:
   DISALLOW_COPY_AND_ASSIGN({{struct.name}});
 {%- endif %}
 };