[iOS] CPE: Save newly generated credentials

The password itself is stored into the keychain, and the Credential
object is saved to NSUserDefaults, so it can be read the next time
Chrome starts.

After saving finishes, the extension also passes the credentials back
to the website, closing itself.

Bug: 1224974
Change-Id: I622954e9b734f07400798de84f6f6ed4d6d0f3da
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3058648


Commit-Queue: Robbie Gibson <rkgibson@google.com>
Reviewed-by: Javier Flores <javierrobles@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#907168}