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The multilogin mode is currently dropped on the floor by the account
reconcilor. This mode needs to be passed to the Gaia server.

This CL passes it to the identity service (by passing the full
MultiloginParameters structure instead of the account list).
A follow-up CL will complete the plumbing to Gaia.

Bug: 890321
Change-Id: I711fed0b26a60b74b976e348111029f6948b187f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1893252Reviewed-by: Monica Basta <msalama@chromium.org>
Commit-Queue: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#712528}