Add webauth makeCredential mojom.

Patch #1 of multiple

This patch adds the webauth authenticator mojom interface. It has the function makeCredential and necessary structs. The function is used by the WebAuthentication class.

BUG=664630

Review-Url: https://codereview.chromium.org/2702653002
Cr-Commit-Position: refs/heads/master@{#471498}
8 files changed