Omnibox: Add back vertical margin field trial

This CL is a partial revert of:
https://chromium-review.googlesource.com/c/chromium/src/+/1257806

It adds back the vertical margin trial, but it no longer ties it to
kExperimentalUi. It's just set to 2 by default, to match the
pre-Refresh look as closely as possible.

This can be manually tested like this:

chrome
  --force-fieldtrials=FakeStudy/FakeGroup
  --force-fieldtrial-params=FakeStudy.FakeGroup:UIVerticalMargin/12
  --enable-features="OmniboxUIExperimentVerticalMargin<FakeStudy"

Bug: 931332
Change-Id: Ic590f736b141a20bb10ee1e4d4f688d18d68910f
Reviewed-on: https://chromium-review.googlesource.com/c/1467345
Commit-Queue: Tommy Li <tommycli@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#631521}
4 files changed