Implement CredentialsContainer::create

This change adds `CredentialsContainer::create` to the Credential Management API
allowing for asynchronous creation of Credential objects.

Intent to Ship:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/uEGfd5NmjbY/IUWw7lC6EAAJ

BUG=722242

Review-Url: https://codereview.chromium.org/2884703002
Cr-Commit-Position: refs/heads/master@{#473197}
16 files changed