Add OPTIMIZATION_TARGET_SEGMENTATION_IOS_DEFAULT_BROWSER_PROMO

This will be a binary classifier ML model queried by the iOS client
to determine whether to show the default browser promo.

Bug: 407788921
Change-Id: I5e557f35461e4c1c6e21bedef08aa8f12270e710
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6619995
Reviewed-by: Ritika Gupta <ritikagup@google.com>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1470134}