Function webkit2gtk_sys::webkit_network_proxy_settings_add_proxy_for_scheme

pub unsafe extern "C" fn webkit_network_proxy_settings_add_proxy_for_scheme(
    proxy_settings: *mut WebKitNetworkProxySettings, 
    scheme: *const c_char, 
    proxy_uri: *const c_char
)

This is supported on crate feature v2_16 only.