LibWeb: Add Contrast preference

Author: https://github.com/lukewarlow 🔰
Commit: ee64684565
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/146
Reviewed-by: https://github.com/AtkinsSJ

Userland/Services/WebContent/PageClient.cpp

@@ -155,6 +155,13 @@ void PageClient::set_preferred_color_scheme(Web::CSS::PreferredColorScheme color
         document->invalidate_style();
 }
 
+void PageClient::set_preferred_contrast(Web::CSS::PreferredContrast contrast)
+{
+    m_preferred_contrast = contrast;
+    if (auto* document = page().top_level_browsing_context().active_document())
+        document->invalidate_style();
+}
+
 void PageClient::set_is_scripting_enabled(bool is_scripting_enabled)
 {
     page().set_is_scripting_enabled(is_scripting_enabled);