Add a provision in WebKitURIResponse to check if the response is from…

[pgorszkowski-igalia]

… a main frame resource

[pgorszkowski-igalia]

Tested with COG on RPi by calling:

WebKitWebResource* resource = webkit_web_view_get_main_resource(web_view);
WebKitURIResponse* response = webkit_web_resource_get_response(resource);

incase of WEBKIT_LOAD_FINISHED