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/values/specified/text.rs

@@ -596,7 +596,7 @@ impl ToComputedValue for TextAlign {
                 // In that case, the default behavior here will set it to left,
                 // but we want to set it to right -- instead set it to the default (`start`),
                 // which will do the right thing in this case (but not the general case)
-                if _context.is_root_element {
+                if _context.builder.is_root_element {
                     return TextAlignKeyword::Start;
                 }
                 let parent = _context