s13n: Add legacy API to replace AccountTrackerService::PickAccountIdForAccount

The AccountTrackerService::PickAccountIdForAccount will be removed when the delegate
is moved inside the component. Until then, we expose the method as
IdentityManager::LegacyPickAccountIdForAccount.

TEST: IdentityManagerTest.TestLegacyPickAccountIdForAccount
Bug: 922471
Change-Id: I9cf97b9899b00868baf7439b50bc8f1b39a8c8c6
Reviewed-on: https://chromium-review.googlesource.com/c/1417402
Commit-Queue: Gyuyoung Kim <gyuyoung@igalia.com>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624572}
3 files changed