Change oauth_multilogin code to not use account_id as gaia_id

The multilogin code assumes that accountID is Gaia ID which is not true
for chromeOS. This cl is the second part of the refactor intended to
change OauthMultiLoginHelper code to get both account id and gaia id
and pass account id to OauthMultiLoginFetcher and gaia_id to GaiaAuthFetcher.

For context the first part of the cl:
https://chromium-review.googlesource.com/c/chromium/src/+/1624189

Bug: 956503
Change-Id: I1805de19aa0466bd00c618fa7eb2db76daf34cab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1660339
Commit-Queue: Tanmoy Mollik <triploblastic@google.com>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#670839}