Starting to work on this.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/fcf7047148f59c25709f63602e015077d2e7c559

commit fcf7047148f59c25709f63602e015077d2e7c559
Author: Julie Jeongeun Kim <jkim@igalia.com>
Date: Tue Jan 15 15:02:43 2019

[s13n] Convert SyncConfirmationHandler to IdentityManager

This CL only migrates SyncConfirmationHandler production code away
from SigninManager.

Bug:  920245 
Change-Id: Ib65d9912c378ff91c8a3b00aacc3a926b6f97016
Reviewed-on: https://chromium-review.googlesource.com/c/1405953
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Cr-Commit-Position: refs/heads/master@{#622857}
[modify] https://crrev.com/fcf7047148f59c25709f63602e015077d2e7c559/chrome/browser/ui/webui/signin/sync_confirmation_handler.cc
[modify] https://crrev.com/fcf7047148f59c25709f63602e015077d2e7c559/chrome/browser/ui/webui/signin/sync_confirmation_handler.h