[Nav Experiment] Implement GetVisibleItem().

Also tweaked DCHECK in CRWWebController because |webViewNavigationProxy|
should return nil instead of triggering DCHECK if the underlying web
view is not set.

Bug: 734150
Change-Id: I83b501de1551aca36014a6f2f304230dab0aac69
Reviewed-on: https://chromium-review.googlesource.com/606115
Commit-Queue: Danyao Wang <danyao@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492754}