This implements the following AOs:
- Create a PasswordCredential from PasswordCredentialData.
- Create a PasswordCredential from an HTMLFormElement.
Which corresponds to these PasswordCredential ctors:
- constructor(PasswordCredentialData)
- constructor(HTMLFormElement)
Previously the attributes for the mixin CredentialUserData was added to
the Credential implementation. Now they are moved to the
CredentialUserData implementation, and any *Credential implementations
which includes this mixin in IDL can now instead inherit from the
CredentialUserData class.
Aligning with the Chromium implementation, deviating from the spec we
remove the 'origin' attribute from FederatedCredentialInit and
PasswordCredentialData.
