Add support for generating external types in ComputedStyleBase.

third_party/WebKit/Source/build/scripts/templates/fields/external.tmpl

+{% macro decl_methods(field) %}
+inline static {{field.type_name}} {{field.initial_method_name}}() {
+  return {{field.default_value}};
+}
+const {{field.type_name}}& {{field.getter_method_name}}() const {
+  return {{field.name}};
+}
+void {{field.setter_method_name}}(const {{field.type_name}}& v) {
+  {{field.name}} = v;
+}
+void {{field.setter_method_name}}({{field.type_name}}&& v) {
+  {{field.name}} = std::move(v);
+}
+inline void {{field.resetter_method_name}}() {
+  {{field.name}} = {{field.default_value}};
+}
+{% endmacro %}