Credential Manager: stub out `navigator.credentials`.

This patch implements NavigatorCredentials, and stubs out a single method
of CredentialsContainer: 'request()'. Currently that method only returns
and resolves a promise. Eventually, it will do something slightly more
exciting and interesting.

BUG=400674

Review URL: https://codereview.chromium.org/449013002

git-svn-id: svn://svn.chromium.org/blink/trunk@179799 bbb929c8-8fbe-4397-9dbb-9b2b20218538