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Building for "catalyst" environment requires some specific gn settings,
so to facilitate the developers' job, add support for this environment
to //ios/build/tools/setup-gn.py.

Remove support for "fat" build as it is unsupported (since deployment
target was bumped to 11.0+).

Bug: 1138425
Change-Id: Ia40cb9a7b72a70e46b1a938285c0a020ea95f619
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517732
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824378}