[s13n] Add LegacyReloadAccountsFromSystem to IdentityManager

This CL adds LegacyReloadAccountsFromSystem to IdentityManager and
OAuth2TokenServiceDelegate. IdentityManager::LegacyReloadAccountsFromSystem
calls the API from OAuth2TokenServiceDelegate and it triggers platform specific
implementations for Android and IOS.

It's only for replacement of Android and IOS production code.

Bug: 926944
Change-Id: I9b18baab93364cf03c113710daf627dd2fa29c0c
Reviewed-on: https://chromium-review.googlesource.com/c/1460640
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Reviewed-by: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631446}
7 files changed