[iOS] Fix progress bar visibility

This CL fixes the progress bar visibility, which could be fooled when
the state was changing too quickly. For example, if the loading state
was changing during the hide animation, the progress bar wasn't shown
again.

Bug: 925598
Change-Id: Ief9d7cff09724467b244fa1aadecc7a41f7ac0ec
Reviewed-on: https://chromium-review.googlesource.com/c/1489238
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635981}