style: Add a style flag for the root element style.

This is needed to make the root element not a containing block in presence of
filters or what not.

Differential Revision: https://phabricator.services.mozilla.com/D61167

components/style/lib.rs

@@ -120,6 +120,8 @@ pub mod attr;
 pub mod author_styles;
 pub mod bezier;
 pub mod bloom;
+#[path="properties/computed_value_flags.rs"]
+pub mod computed_value_flags;
 pub mod context;
 pub mod counter_style;
 pub mod custom_properties;