Bring the two functions closer together in implementation. Avoid
unnecessary assignment to top_on_previous_step_.

Bug: 
Change-Id: I0b6881b02ab08a48bc2051e6786e22a7b25a4948
Reviewed-on: https://chromium-review.googlesource.com/815196Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Ali Ijaz Sheikh <ofrobots@google.com>
Cr-Commit-Position: refs/heads/master@{#49944}