IMPORTANT:
- This launch issue is used for standards and implementation tracking only.
It is NOT for Chrome approval regarding privacy, security, legal, UI,
testing,
accessibility etc.
- An overview of the different launch issues can be found here:
http://bit.ly/2ncKZfp
- If your feature requires security / privacy / etc approval then seek a PM
to guide you through the go/newchromefeature process

See http://www.chromium.org/blink#launch-process for an overview of the
Blink launch process, but note this doesn't cover security / privacy / etc
reviews as discussed above.

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Change description:
This change introduces `CredentialsContainter::create` to the Credential Manager API, which allows for asynchronous creation of Credentials.

Changes to API surface:
- Implement `CredentialsContainer::create`

Links:
Public standards discussion: https://github.com/w3c/webappsec-credential-management/issues
Relevant Spec Section: https://w3c.github.io/webappsec-credential-management/#dom-credentialscontainer-create

Support in other browsers:
Internet Explorer: NA
Firefox: NA
Safari: NA