GitLab

Cleans up the APIs to show the previews UI by abstracting away the
InfoBar and leaving the Previews Tab Helper as the single point of
entry to show a Previews UI.

Also renames and moves the on dismiss callback so that it no longer
has "infobar" in its name.

Bug: 874523
Change-Id: I49735c52882d198cd9f6fe95725168691a88681d
Reviewed-on: https://chromium-review.googlesource.com/1176544Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583766}