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

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

 {#---
   Macro for enum definition, and the declaration of associated functions.
 ---#}
 
 {%- macro enum_decl(enum) %}
 {%-   set enum_name = enum|get_name_for_kind(flatten_nested_kind=True) %}
 enum class {{enum_name}} : int32_t {
 {%-   for field in enum.fields %}
 {%-     if field.value %}
   {{field.name}} = {{field.value|expression_to_text}},
@@ skipping 27 matching lines @@
   return {{enum|get_name_for_kind(internal=True,
                                   flatten_nested_kind=True)}}::IsKnownValue(
       static_cast<int32_t>(value));
 }
 {%- endmacro %}
 
 {%- macro enum_data_decl(enum) %}
 {%-   set enum_name = enum|get_name_for_kind(flatten_nested_kind=True) %}
 struct {{enum_name}}_Data {
  public:
-  static bool const kIsExtensible = {% if enum.extensible %}true{% else %}false{% endif %};
+  static bool constexpr kIsExtensible = {% if enum.extensible %}true{% else %}false{% endif %};
 
   static bool IsKnownValue(int32_t value) {
 {%- if enum.fields %}
     switch (value) {
 {%-   for enum_field in enum.fields|groupby('numeric_value') %}
       case {{enum_field[0]}}:
 {%-   endfor %}
         return true;
     }
 {%- endif %}
@@ skipping 12 matching lines @@
 };
 {%- endmacro %}
 
 {%- macro enum_hash(enum) %}
 {%-   set enum_name = enum|get_qualified_name_for_kind(
           flatten_nested_kind=True) %}
 template <>
 struct hash<{{enum_name}}>
     : public mojo::internal::EnumHashImpl<{{enum_name}}> {};
 {%- endmacro %}