[omnibox] Change OmniboxFieldTrial from class to namespace

OmniboxFieldTrial has no member data or functions. It's just a scope,
to bundle Omnibox feature switches in. This CL makes that official.

Change-Id: Ib4b98417bc10ba6b8acedc2c23bb0b2d97d99e7e
Reviewed-on: https://chromium-review.googlesource.com/c/1472856
Commit-Queue: Kevin Bailey <krb@chromium.org>
Reviewed-by: Justin Donnelly <jdonnelly@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633364}
3 files changed