save
Description:
Asynchronously save the current web page.
Asynchronously save the current web page associated to the WebView into a 
      self-contained format using the mode specified in save_mode.
When the operation is finished, callback will be called. You can then call save.end to get the result 
      of the operation.
Parameters:
| this | a WebView | 
| save_mode | the SaveMode specifying how the web page should be saved. | 
| 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 |