[Autofill Assistant] Improve CounterInput UI.

 * Before: https://screenshot.googleplex.com/hmVvTEU3Bg7.png
 * After: https://screenshot.googleplex.com/Cp3bg0Wqt7m.png

Screenshots: 
Change-Id: I3c3563eb895fde6a0fd86ab25625eb68aa0ce598
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1617828
Reviewed-by: Mathias Carlen <mcarlen@chromium.org>
Commit-Queue: Jordan Demeulenaere <jdemeulenaere@chromium.org>
Cr-Commit-Position: refs/heads/master@{#661345}
13 files changed