Define only webkit_web_extension_initialize_with_user_data
While searching whether some other applications will be affected by new behavior of WebKitGTK+ (in 2.26, where multiple WebkitWebProcess-es can be created unconditionally for a single WebKitWebContext and there can be multiple WebProcess-es referencing the same WebKitWebPage WebKitGTK+ bug), I noticed that you declare both
webkit_web_extension_initialize_with_user_data. Do not do that, declare only one of them. You are rather lucky that they try the
webkit_web_extension_initialize_with_user_data first and the other one the second time.