[Omnibox] Add logging for steady state omnibox position

Add logging for the preferred steady state (unfocused) omnibox position.
On iOS users can choose between top and bottom omnibox. Log the omnibox
position in omnibox event to measure the impact on omnibox usage.

Bug: 1468484
Change-Id: Ib219fee197be577d541dc3deb143ad587d1e80b8
Low-Coverage-Reason: Added code are covered by tests.
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4738317
Reviewed-by: manuk hovanesian <manukh@chromium.org>
Auto-Submit: Christian Xu <christianxu@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1179563}