[iOS] Fix alignment of Keyboard Accessory View buttons.

The KAV buttons should be aligned with the ones from GBoard.

Bug: 
Change-Id: I8ba048719326ed93890fa989092cc460d0071e29
Reviewed-on: https://chromium-review.googlesource.com/579971


Commit-Queue: Jean-François Geyelin <jif@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488324}