GitLab

Don't need to store the previous_url, NavigationHandle does it for us.

This removes 1 copy on navigation start and 4 copies on commit. It
also simplifies the code a reasonable amount.

Bug: None
Change-Id: I1f19f87a254c6b6fb5147a38da23f66b5db55c01
Reviewed-on: https://chromium-review.googlesource.com/774985Reviewed-by: Martin Šrámek <msramek@chromium.org>
Commit-Queue: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#517499}