CL #1 (migrates SigninManager -> IdentityManager): https://crrev.com/c/1318550.

For migrating away from PO2TS, it needs to have an IdentityManager replacement for  OAuth2TokenService::UpdateCredentials() available -  bug 887870 .

Also blocking bug 890790 (Convert c/b/signin/dice_response_handler_unittest.cc to IdentityManager).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f4a5aa2a645ba65b07940d9d77552b384b9b600a

commit f4a5aa2a645ba65b07940d9d77552b384b9b600a
Author: Antonio Gomes <tonikitoo@igalia.com>
Date: Thu Nov 08 13:12:50 2018

[s13n] Convert DiceResponseHandler to IdentityManager

This CL only migrates DiceResponseHandler production code away from
SigninManager, leaves migration away from ProfileOAuth2TokenService
and unittests for a follow up step.

Note that the unittest c/b/signin/dice_response_handler_unittest.cc
was minimally updated to keep passing.

BUG=890790, 902296 

Change-Id: Iecdf43b80afe6514b7f19075c2bb62aadfbdbe4c
Reviewed-on: https://chromium-review.googlesource.com/c/1318550
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#606439}
[modify] https://crrev.com/f4a5aa2a645ba65b07940d9d77552b384b9b600a/chrome/browser/signin/dice_response_handler.cc
[modify] https://crrev.com/f4a5aa2a645ba65b07940d9d77552b384b9b600a/chrome/browser/signin/dice_response_handler.h
[modify] https://crrev.com/f4a5aa2a645ba65b07940d9d77552b384b9b600a/chrome/browser/signin/dice_response_handler_unittest.cc

Missing bits: https://chromium-review.googlesource.com/c/chromium/src/+/1412118

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3eb99abcaae8dd125fe03fe16b19b3a5fc03abcc

commit 3eb99abcaae8dd125fe03fe16b19b3a5fc03abcc
Author: Antonio Gomes <tonikitoo@igalia.com>
Date: Fri Jan 18 15:50:32 2019

[s13n] Convert DiceResponseHandler away from PO2TS

This is the 2nd out of two CLs that convert DiceResponseHandler
away from SigninManager and PO2TS APIs, in favor of IdentityManager's.
Particularly, this tackles the later (former was handled here [1]).

[1] https://crrev.com/c/1318550

BUG= 902296 

Change-Id: I64ec0eee99a30d419dd46dabbd3c0396e478ce95
Reviewed-on: https://chromium-review.googlesource.com/c/1412118
Commit-Queue: Antonio Gomes <tonikitoo@igalia.com>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Reviewed-by: David Roger <droger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624138}
[modify] https://crrev.com/3eb99abcaae8dd125fe03fe16b19b3a5fc03abcc/chrome/browser/signin/dice_response_handler.cc
[modify] https://crrev.com/3eb99abcaae8dd125fe03fe16b19b3a5fc03abcc/chrome/browser/signin/dice_response_handler.h
[modify] https://crrev.com/3eb99abcaae8dd125fe03fe16b19b3a5fc03abcc/chrome/browser/signin/dice_response_handler_unittest.cc