Credential Manager: Drop 'optional' from notify* |credential| parameters.

At some point, we'd like these to be optional. At the moment, though,
they're not. Once we finish hammering out the functionality in the spec
discussions, we can come back and implement this behavior properly. For
the moment, we shouldn't crash. :)

BUG=415450

Review URL: https://codereview.chromium.org/584903002

git-svn-id: svn://svn.chromium.org/blink/trunk@182346 bbb929c8-8fbe-4397-9dbb-9b2b20218538