LibWebView+WebContent: Inform WebContent process if browser is headless

Author: https://github.com/tcl3
Commit: e764df15eb
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/2858
Reviewed-by: https://github.com/trflynn89

Libraries/LibWeb/Page/Page.h

@@ -407,6 +407,8 @@ public:
 
     virtual DisplayListPlayerType display_list_player_type() const = 0;
 
+    virtual bool is_headless() const = 0;
+
 protected:
     virtual ~PageClient() = default;
 };