Credential Manager: basic Credential interfaces

This is step towards a stub implementation of the Credential Manager API
defined in http://projects.mikewest.org/credentialmanagement/spec/. For
obvious reasons, it isn't web-exposed, and isn't yet enabled by the
experimental flag. There's still a good amount of work to do before the
API actually _does_ anything, much less anything we'd like folks to
start banging on.

BUG=400674

Review URL: https://codereview.chromium.org/450633002

git-svn-id: svn://svn.chromium.org/blink/trunk@179727 bbb929c8-8fbe-4397-9dbb-9b2b20218538