style: Kill SharedStyleContext::default_computed_values.

Now that cascade() gets a Device, we can use the default computed values from
there to avoid propagating that state all over the place.

Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>

components/style/matching.rs

@@ -543,7 +543,6 @@ trait PrivateMatchMethods: TElement {
                              rule_node,
                              inherited_values,
                              layout_parent_style,
-                             &shared_context.default_computed_values,
                              Some(&mut cascade_info),
                              shared_context.error_reporter.clone(),
                              cascade_flags));