style: Don't look for viewport units in stylesheets.

Use whether we've computed any viewport unit instead.

This is more accurate (we avoid restyling unnecessarily if we've found anything
ever on the stylesheet, but that hasn't matched).

This has the benefit of also matching Gecko, and simplify some code and
fishyness around, and also hopefully speeding up stylesheet parsing.

components/script/stylesheet_loader.rs

@@ -289,7 +289,6 @@ impl<'a> StyleStylesheetLoader for StylesheetLoader<'a> {
                 rules: CssRules::new(Vec::new(), lock),
                 origin: context.stylesheet_origin,
                 url_data: RwLock::new(context.url_data.clone()),
-                dirty_on_viewport_size_change: AtomicBool::new(false),
                 quirks_mode: context.quirks_mode,
                 namespaces: RwLock::new(Namespaces::default()),
                 source_map_url: RwLock::new(None),