[ios] Do not build user agent utils unittest

These are not currently supported on the platform.

Bug: 1406229
Change-Id: I4f0a601eb2bfb58031a36b07b852b6478bef0398
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4350334


Reviewed-by: Mike Taylor <miketaylr@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118920}