LibWeb/CSS: Set @import as the owner rule of their loaded style sheet

Author: https://github.com/AtkinsSJ
Commit: https://github.com/LadybirdBrowser/ladybird/commit/240b58e90fb
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/1168
Reviewed-by: https://github.com/trflynn89

Userland/Libraries/LibWeb/CSS/CSSImportRule.cpp

@@ -104,6 +104,7 @@ void CSSImportRule::resource_did_load()
     }
 
     m_style_sheet = sheet;
+    m_style_sheet->set_owner_css_rule(this);
 
     m_document->style_computer().invalidate_rule_cache();
     m_document->style_computer().load_fonts_from_sheet(*m_style_sheet);