[s13n] Add LegacySeedAccountInfo to IdentityManager

This CL adds LegacySeedAccountInfo to IdentityManager to replace
AccountTrackerService::SeedAccountInfo in production code.

Test: IdentityManagerTest.LegacySeedAccountInfo
Bug: 922473
Change-Id: Ie0107dd36db6e53da851d8b1f19606f2e712f704
Reviewed-on: https://chromium-review.googlesource.com/c/1416792
Commit-Queue: Julie Jeongeun Kim <jkim@igalia.com>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624711}
3 files changed