Reland "[ios] add credit card test for manual fallback"

This is a reland of 30e1dd180408413c8fce6dbe5b856c048a8ca908

Adding feature flags to slimnav.
Revert bug: https://bugs.chromium.org/p/chromium/issues/detail?id=908970

Original change's description:
> [ios] add credit card test for manual fallback
>
> Also adds AutofillManualFallbackPhaseTwo to test bots.
>
> Bug: 845472
> Change-Id: I6bd05781c09c24c0768e24acfe59efa7ce1da05d
> Reviewed-on: https://chromium-review.googlesource.com/c/1337347
> Reviewed-by: Rohit Rao <rohitrao@chromium.org>
> Reviewed-by: Ben Pastene <bpastene@chromium.org>
> Reviewed-by: Javier Ernesto Flores Robles <javierrobles@chromium.org>
> Commit-Queue: David Jean <djean@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#611171}

TBR=rohitrao@chromium.org

Bug: 845472,908970
Change-Id: I2cc0cddf0be8cea447f45f69457afbe19c840f29
Reviewed-on: https://chromium-review.googlesource.com/c/1352328
Commit-Queue: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Reviewed-by: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611679}
6 files changed