Another API in use that needs migrating is this:

  * SigninManager::GetAuthenticatedAccountId() --> maps to IdentityManager::GetPrimaryAccountId()

Also, note some part of the work required here will be done as part of the CLs for  crbug.com/809440  because of adapting usage of drive::DriveAPIService(), which needs to be adapted because of migrating AuthService away from OAuth2TokenService.

CL here: https://chromium-review.googlesource.com/c/chromium/src/+/1297981

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/dd88196c0da623f918f2c80ddf0d8177b11834d0

commit dd88196c0da623f918f2c80ddf0d8177b11834d0
Author: Mario Sanchez Prada <mario@igalia.com>
Date: Wed Nov 07 19:08:20 2018

Migrate the SyncEngine drive backend to the IdentityManager

Remove all trace of SigninManager from SyncEngine, which also
required some changes in clients and unit tests due to the removal
of one parameter from the SyncEngine's constructor.

Bug:  890798 
Change-Id: I10596e61c46f7b86b8afb37c2dbfc401f5f651a8
Reviewed-on: https://chromium-review.googlesource.com/c/1297981
Commit-Queue: Mario Sanchez Prada <mario@igalia.com>
Reviewed-by: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606096}
[modify] https://crrev.com/dd88196c0da623f918f2c80ddf0d8177b11834d0/chrome/browser/apps/platform_apps/api/sync_file_system/sync_file_system_browsertest.cc
[modify] https://crrev.com/dd88196c0da623f918f2c80ddf0d8177b11834d0/chrome/browser/sync_file_system/drive_backend/drive_backend_sync_unittest.cc
[modify] https://crrev.com/dd88196c0da623f918f2c80ddf0d8177b11834d0/chrome/browser/sync_file_system/drive_backend/sync_engine.cc
[modify] https://crrev.com/dd88196c0da623f918f2c80ddf0d8177b11834d0/chrome/browser/sync_file_system/drive_backend/sync_engine.h
[modify] https://crrev.com/dd88196c0da623f918f2c80ddf0d8177b11834d0/chrome/browser/sync_file_system/drive_backend/sync_engine_unittest.cc