get_accept_policy
Description:
Asynchronously get the cookie acceptance policy of this.
Note that when policy was set to webkit_cookie_policy_accept_no_third_party and ITP is enabled, this will 
      return webkit_cookie_policy_accept_always. See also webkit_website_data_manager_set_itp_enabled.
    
When the operation is finished, callback will be called. You can then call get_accept_policy.end to 
      get the result of the operation.
Parameters:
| this | |
| cancellable | a Cancellable or null to ignore | 
| callback | a TaskReadyCallback to call when the request is satisfied | 
| user_data | the data to pass to callback function |