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)
Before this change, we were going through the chain of base classes for
each IDL interface object and having them set the prototype to their
prototype.
Instead of doing that, reorder things so that we set the right prototype
immediately in Foo::initialize(), and then don't bother in all the base
class overrides.
This knocks off a ~1% profile item on Speedometer 3.
