LibWeb: Invalidate styles after CSSImportRule loads

This replicates the behavior of StyleSheetList::add_sheet, making sure
the rules added by the imported style sheet are applied.

Userland/Libraries/LibWeb/CSS/CSSImportRule.cpp

@@ -84,6 +84,9 @@ void CSSImportRule::resource_did_load()
     }
 
     m_style_sheet = move(sheet);
+
+    m_document->style_sheets().bump_generation();
+    m_document->invalidate_style();
 }
 
 }