SigninManager should get LST from credential provider when possible

Bug: 907882
Change-Id: Icaf7f785bf05ab24ab098e91453df9d1fe787295
Reviewed-on: https://chromium-review.googlesource.com/c/1347787
Reviewed-by: David Roger <droger@chromium.org>
Commit-Queue: Roger Tawa <rogerta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616678}
18 files changed