[omnibox] Add field trial testing config for starter pack expansion.

Bug: 41494524
Change-Id: Ic44c27868876025862488bb2f3d4826d38cd8e24
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5388891


Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Auto-Submit: Angela Yoeurng <yoangela@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1277113}