WebContent+LibWebView: Add support for user style sheets

Author: https://github.com/AtkinsSJ
Commit: ec340f03a5
Pull-request: https://github.com/SerenityOS/serenity/pull/20699

Userland/Libraries/LibWebView/OutOfProcessWebView.h

@@ -54,6 +54,8 @@ public:
     // In practice, this means that OOPWV may render scaled stale versions of the content while resizing.
     void set_content_scales_to_viewport(bool);
 
+    void set_user_style_sheet(String source);
+
 private:
     OutOfProcessWebView();