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/stylesheets/viewport_rule.rs

@@ -733,7 +733,6 @@ impl MaybeNew for ViewportConstraints {
 
         let mut conditions = RuleCacheConditions::default();
         let context = Context {
-            is_root_element: false,
             // Note: DEVICE-ADAPT § 5. states that relative length values are
             // resolved against initial values
             builder: StyleBuilder::for_inheritance(device, None, None),