[Dice] Implement Dice migration flow

When the migration is in progress, the AccountReconcilor is partially
enabled: it lists the Gaia accounts and compares them against Chrome
accounts. If the reconciliation is a no-op (i.e. the accounts are
consistent), the profile is migrated to Dice.
If the accounts are not consistent, the reconcilor just returns without
touching the accounts.

Other specific behavior when the migration is in progress:
- on Gaia signin, tokens are added to the token service (but the
  Sync promo is not shown)
- on Gaia signout tokens are deleted from the token service, except
  the main account (signing out of Gaia cannot turn off Sync)
- the Gaia signout confirmation dialog is never shown

TBR: bauerb, skym
Bug: 771908
Change-Id: Ia8c21046e1c9a3a745153b5a1204b6ee1271e71e
Reviewed-on: https://chromium-review.googlesource.com/720815
Commit-Queue: David Roger <[email protected]>
Reviewed-by: Bernhard Bauer <[email protected]>
Reviewed-by: Mihai Sardarescu <[email protected]>
Cr-Commit-Position: refs/heads/master@{#511811}
22 files changed