[Extensions] Add experiment config for UseItemSnippetsAPI

This adds testing config for the feature implemented in
crrev.com/c/5246130 for Windows, Mac and Linux

Migrating tests for ChromeOS so they pass with the new feature flag
will be done in a follow up

Bug: 325314721
Change-Id: I297d61e947caf65f8b90b683161d3e27c289b658
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5610474


Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Commit-Queue: Kelvin Jiang <kelvinjiang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313052}