Asynchronously run script in the context of the current page in this.
If WebKitSettings:enable-javascript is FALSE, this method will do nothing.
When the operation is finished, callback will be called. You can then call run_javascript.end to
get the result of the operation.
| this |
a WebView |
| script |
the script to run |
| cancellable |
a Cancellable or null to ignore |
| callback |
a TaskReadyCallback to call when the script finished |
| user_data |
the data to pass to callback function |