Fix scroll positioning logic for BiDirection omnibox text.

BUG=881659

Change-Id: Ib1ad439d73b7951603a35fb9760d9cec943f0acd
Reviewed-on: https://chromium-review.googlesource.com/c/1257666
Commit-Queue: Ted Choc <tedchoc@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Reviewed-by: Troy Hildebrandt <thildebr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#596036}