[ios] disable manual fallback in incognito mode

Right now two formInputAccessoryViews are (lazily) created for normal and incognito,
and the second one created (in this case incognito) remains on top when going back to the other mode.

This CL disables the incognito one until it can be fixed properly.

Bug: 923857
Change-Id: I97e56dffe66a1864aad028df28870948c9f28874
Reviewed-on: https://chromium-review.googlesource.com/c/1429041
Reviewed-by: Mark Cogan <marq@chromium.org>
Reviewed-by: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: David Jean <djean@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625567}