GitLab

Add warning level logging when:
- It's detected that update_engine is downloading a rollback
  image or switching channel.
- Notification about the rollback or channel switch to a more
  stable channel is shown.

Bug: 871759
Change-Id: I930b7a23def82ff239ec8da4feeff9daaf960b37
Reviewed-on: https://chromium-review.googlesource.com/1190882
Commit-Queue: Marton Hunyady <hunyadym@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586754}