Add field trial config for default browser experiment

Bug: 332370957
Change-Id: I7e82d01029ab9c3b1fd4bbcd85e9fc79a6eb2e90
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5415915


Commit-Queue: Alison Gale <agale@chromium.org>
Reviewed-by: Eshwar Stalin <estalin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1282719}