Implement PasswordCredential

[AxelNennker]

As a website developer I want to create a new PasswordCredential object by writing e.g.:

var credential = new PasswordCredential("JaneDoe98","MfPeRQq5P3yVry68Q4KZMMhB");

Please note the difference to the current spec which requires an objects as parameter:

var credential = new PasswordCredential({
  "id": "JaneDoe98",
  "password": "MfPeRQq5P3yVry68Q4KZMMhB"
});

This is not possible with the current WebIDL.